All Replies on Help - it's raining rust on my table saw!

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View endgrainy's profile

Help - it's raining rust on my table saw!

by endgrainy
posted 06-29-2014 07:01 PM

22 replies so far

View TheWoodenOyster's profile


1335 posts in 2908 days

#1 posted 06-29-2014 07:41 PM

Hey there. I can tell you right off, I seriously doubt that you’ll be able to have the metal removed. That metal is probably what your slab is poured directly on. That is called pan decking and it is what concrete is usually poured on top of, so removing the metal is probably not really an option. In other words, it is structural.

That said, you could try the epoxy floor, but that sure is a lot of work unless you want to do it anyway.

This is what I would do – Get an angle grinder and put on some flap discs (sanding discs) and go around and sand all of the rusty spots and bubbly spots that look like they will rust. Get them down to bare metal and then spray them with cold galvanizing spraypaint. This spray is basically an after the fact galvanizing operation used on things like outdoor welds that would rust if not protected. Typically, the moisture is coming from the painted side, but in your case it sounds like it might be coming from above. So the paint might not work, but either way it is worth a shot and is easier than doing an epoxy floor. It might work, you’ll just have to give it a shot. Good Luck!!!

-- The Wood Is Your Oyster

View endgrainy's profile


251 posts in 2861 days

#2 posted 06-29-2014 07:52 PM

Thanks oyster, that sounds like a great idea. I’m thinking about having the garage floor epoxied anyway, but probably not until the fall.

I like a your idea of cold galvanizing spray paint – that’s the kind of thing I wouldn’t know existed, I’m thankful for the expertise. Plus, your method gives me an excuse to get an angle grinder! Would you recommend getting sanding discs for metal? Any particular grit? (I have no experience with metal)

-- Follow me on Instagram @endgrainy

View bigblockyeti's profile (online now)


6959 posts in 2694 days

#3 posted 06-29-2014 08:01 PM

It would be expensive, but you can have two part foam insulation sprayed right over the metal as is, or you can clean some of the rust off a bit before having it sprayed. It would keep condensation from being able to form on the metal and wouldn’t add much dimension to the ceiling cutting down on your head room. Sanding the rust off the metal could be accomplished efficiently with a grit in the 60 – 80 area.

-- "Lack of effort will result in failure with amazing predictability" - Me

View endgrainy's profile


251 posts in 2861 days

#4 posted 06-29-2014 08:11 PM

Good thought, thanks bigblockyeti. The insulation would be good for my long term plans of finishing off the room one day. I’m wondering if it would be bad though to seal on both sides of the metal pan decking – top sealed with epoxy over the concrete, bottom with spray foam – the worry being that rust would continue unseen and one day the rusted floor would give out.

However, I guess if there’s no oxygen and no new moisture then no further rust would occur.

And thanks for the grit recs.

-- Follow me on Instagram @endgrainy

View William's profile


9950 posts in 3815 days

#5 posted 06-29-2014 08:14 PM

Can you screw into the metal?
If you can, how about ripping 2×4s in half, stripping it out and hanging exterior grade plywood?
I would say sheet rock, but you have stated a condensation problem, which would probably destroy the sheet rock in short order.
Anyway, the ex-ply could be painted a bright white color that would reflect light real well, which for me (lighting) is always another issue to contend with.


View endgrainy's profile


251 posts in 2861 days

#6 posted 06-29-2014 08:20 PM

Thanks William. Ideally it would be nice to have something other than a metal ceiling and reflective white plywood would be much nicer than my current ceiling. I can screw into the metal, I’ve attached light fixtures into it.

My only concern with finishing with wood/plywood is the ceiling height. It’s about 7’ as it stands currently, not a lot of room to lose. I have thought about moving the floor downwards, but that seems expensive :)

-- Follow me on Instagram @endgrainy

View SCOTSMAN's profile


5849 posts in 4558 days

#7 posted 06-29-2014 09:26 PM

I had the same problem with my new metal storage shed ,then I was told to stick bubble wrap bought in big rolls to the underside of the ceiling.Now since then I have had no condensation problems. It doesn’t look too good but you could also fit a false ceiling with thin tongue and groove pine (treated) with insulation behind it.The problem comes when the hot air or water vapour inside your shop hits the cold steel and turns to water which drips down onto everything.Have fun. Alistair

-- excuse my typing as I have a form of parkinsons disease

View endgrainy's profile


251 posts in 2861 days

#8 posted 06-30-2014 01:07 AM

Thank Alistair, novel solution. I’m glad someone else I’m the world has had a similar problem – multiple google searches turned up nothing.

-- Follow me on Instagram @endgrainy

View mramseyISU's profile


594 posts in 2519 days

#9 posted 06-30-2014 01:16 AM

Hit it with some POR coating.

The stuff works wonders.

-- Trust me I'm an engineer.

View bobkberg's profile


440 posts in 4046 days

#10 posted 06-30-2014 04:10 AM

I like the suggestion about the 2 part spray-on foam. That stuff is REALLY tenacious! Then, once it’s in place, you can paint the ceiling bright white, although I’d recommend a coat of Zinsser 1-2-3 or Kilz first.

-- Bob - A sideline, not how I earn a living

View AnonymousRequest's profile


861 posts in 2522 days

#11 posted 06-30-2014 04:56 AM

What oyster said. I’ve used cold galvanizing paint for years as a primer. Is moisture a problem within your room, maybe a de-humidifier.

View johnstoneb's profile


3165 posts in 3146 days

#12 posted 06-30-2014 01:40 PM

Do what oyster said. Clean the rust sots down to bare metal and repaint. The metal is serving as a moisture barrier, epoxy on the garage floor will do nothing for you other than a nice looking garage floor. Your moisture is coming up thru the floor and condensing on the ceiling. A dehumidifier or spray on insulation as suggested will be your best solutions to stop further moisture and rust after you clean and paint.

-- Bruce, Boise, ID

View TheWoodenOyster's profile


1335 posts in 2908 days

#13 posted 06-30-2014 01:46 PM

For grit, go with the lowest available, 60 or 80, whatever they have. If you are looking for a bargain, go to harbor freight. Their grinders works just fine and are cheeeeep, as are the sanding discs. Also, make sure you wear a respirator down there while you are working. That galvanizing spray is no good for the lungs. Welders have died from breathing that stuff in too much, so heed the warning on the can. Good Luck!

-- The Wood Is Your Oyster

View chrisstef's profile


18127 posts in 3980 days

#14 posted 06-30-2014 02:02 PM

Ive seen a lot of commercial construction projects use a SIKA paint product as a rust inhibitor for metal decking as well as exposed rebar in concrete slabs.

From a little bit of reading it seems that the high alkalyds (sp) in concrete contribute to the rusting of metals. Id be willing to bet that the existing decking that’s rusting in your shop is galvanized as well. You may want to make sure that any new product you use will bond with the galvanized metal.

-- Its not a crack, its a casting imperfection.

View endgrainy's profile


251 posts in 2861 days

#15 posted 06-30-2014 10:41 PM

Thanks for all the advice everyone – great suggestions!

I have ordered some of the cold galvanized spray paint as well as one of the POR products suggested above. I’m going to test in a few areas before I decide what to do on the majority of the ceiling. I see a harbor freight grinder for $25 – that store is almost absurdly inexpensive. I will most definitely use a respirator and try to get airflow moving from the garage down through an open basement door + fans.

If this works short term, maybe I’ll consider having the spray foam option / painting. A dehumidifier would be a welcome addition to the shop – it will also help with comfort (current shop RH is about 70%)

Thanks again to all, I’ll post again in a few weeks once I try some of the suggestions out and see how they’re going.

-- Follow me on Instagram @endgrainy

View John_'s profile


251 posts in 2679 days

#16 posted 07-01-2014 02:46 PM

Just a note about the POR-15 (which really works great by the way). You do not want the TOP COAT, you want the “POR-15 Rust Preventive Coating” which is sensitive to UV light.

I would even consider their roll own bed liner

View generic's profile


105 posts in 2572 days

#17 posted 07-01-2014 05:15 PM

After reading through this post and all the suggested solutions I have to ask. How come you don’t install a dehumidifier? It would help with your machine rust issue and should alleviate you current issue of dripping rust. I would also think that it would be good the stability of your lumber.

My shop in in my basement which has stone and dirt walls. I ended up installing a portable air conditioner that has a dehumidification mode. It is also nice during hot stretches to cool it down to make working more comfortable.

View endgrainy's profile


251 posts in 2861 days

#18 posted 07-02-2014 12:23 AM

Thanks John_H. After reading the POR website, the rust preventing product you linked to is the one I ended up with, not the topcoat.

Thanks generic. A few have mentioned the dehumidifier option, which I think makes sense. The shop is open to the garage via steps (no door) so it might be a challenge for the machine, but it’s worth a shot. I don’t think it would hurt. I do like the portable air conditioner option :)

-- Follow me on Instagram @endgrainy

View CaptainKlutz's profile


4004 posts in 2467 days

#19 posted 07-02-2014 11:43 AM

You have several challenges to work around in a below grade shop open to outside air. I had a partially exposed below grade basement that looked just like that under the garage (except mine had a door).

Others have suggested several methods to fix the rusting metal, and they will work to temporarily fix the problem. But you will need to address the root of the issue – the amount of moisture and the temperature difference causing the condensation.

If you don’t have a way to control airflow (humidity) into/out of the shop; then localized air conditioning or dehumidification is losing battle, plus a waste of electricity (IMHO). If a normal door is not possible, a plastic flap or “strip door” would be an option worth considering in conjunction with the dehumidifier? This reduces the addition of the weather based moisture changes, but I’m guessing you need to reduce how much is generated and retained in you shop as well based on the pictures. It appears you have the standard “floating” concrete floor with cinder block walls in your shop.

You have 2 challenges to moisture control with this construction. First, cinder black is porous, so porous that water flows right through and is commonly used as a gross filter medium in cisterns. IMHO – It absolutely needs to be sealed to reduce the amount of moisture vapor you get from the ground. Second, moisture can penetrate at the expansion joints at the floor/wall intersection as well a little through the concrete floor itself. With the very wet spring seen in most of the mid-west and east this year, I am not surprised it is worse this year, and would guess you might even have some damp spots at the bottom of your block walls that are 7-8 ft below grade? I suggest the basement walls will need to be sealed with a concrete paint like “Drylok, and the floor with an epoxy paint, to help reduce the vapor transfer and minimize moisture retention in the shop. It is not a cure all, but sealing these surfaces will then create a space with the majority of the moisture available to condense is due changes in humidity and not related ground moisture levels. Sealing the walls with Drylok will also brighten things up and get you one step closer to the “finished” shop you mentioned. I should caution you that if you seal the basement floor it will no longer absorb condensation, and can create a slick wet surface (had several falls to prove it) if moisture is not controlled.

Along with moisture control, the next challenge is the temperature differences that are creating the condensation and rusting. It should be obvious that the metal/concrete structure is cooler than the surrounding surfaces, and when the differential reaches the dew point, you get condensation and eventually rusting. The same is true for your tools in the shop area. There are several ways you can address this problem, with adding simple ceiling insulation layer probably being the easiest. I agree with others that suggested using a spray on foam is a good solution. It will slow the permeation of the moisture to the metal, but not completely stop it. You will need to remove the rust and then seal/primer the metal, before you apply the foam to avoid more rust and peeling foam. But as little as 1” of spray foam will insulate the colder metal/concrete from the air, and seriously reduce condensation. Adding 2.5” of closed cell foam insulation to the walls and ceiling would make an energy efficient living space in most states!
Another OK temporary solution, but much more costly long term solution (without a door) is to add localized heating to the metal ceiling to keep it above the dew point. I don’t suggest heating a mass as large as garage floor, but running a space heater or heating tape along the ceiling that is set above typical dew point would be a possible solution to reducing the number of condensation events.

If you go with the spray on foam solution, suggest you use a closed cell 2 part system applied by a professional. You might even find an outfit with the equipment to sand blast all the metal, seal it, and then insulate.

FWIW – The one part pre-catalyzed foams in the spray can from the big box stores are typically open cell varieties that are expensive, not as durable, and allow more water vapor transmission (which is bad idea on a metal base substrate). For the hard core DIY doing his whole house there are a few places that will rent out the special 2 part mixing spray guns, when you buy pails or drums of the chemicals from them. You can also buy kits with pressurized containers, but they cost more per sq ft to apply. Google can tell you more about the options available, which varies based on local EPA regulations .

Best of luck on improving your shop!

-- If it wasn't for bad luck, I wouldn't have no luck at all, - Albert King - Born Under a Bad Sign released 1967

View Hammerthumb's profile


3081 posts in 2948 days

#20 posted 07-02-2014 10:55 PM

It looks like the pan decking is not vented (they do make them vented as well as non-vented). The temp differential between your garage and shop is creating the condensation. De-humidifier would help, but I suggest you call a waterproofing company who is familiar with these systems, or a concrete company that specializes in slabs poured on pan decking. They might have a solution of a more permanent nature than coatings or de-humidifiers.

-- Paul, Duvall, WA

View endgrainy's profile


251 posts in 2861 days

#21 posted 07-03-2014 12:59 AM

Thanks CaptainKlutz – extremely helpful post, thanks for taking the time. As part of shop improvement, I have planned to eventually waterproof the cinder blocks with Drylock or equivalent. As you suggested, I do have damp appearing cinder blocks lower on the walls. I had assumed the moisture was coming from water transfer from the slab above, I now believe it’s coming from multiple sources as you, Hammerthumb, and others above have suggested.

The dew point concept makes sense to me, thanks for explaining it that way. After my rust removal project, I’ll look into some longer terms methods of preventing moisture from accumulating on the metal. Ideally, over a few year period, I’d like to waterproof the walls/floor, treat the above slab (epoxy or similar), and permanently fix the ceiling (spray foam insulation seems like a good option.) One day installing a door to the garage steps and some climate control would be great. Then walls and new (wooden) floor. Then a coffee bar :)

It seems like a long to-do list to improve a difficult space. But there are several upsides of this location: dust is kept out of the house / house HVAC; noise transmission is minimal; it’s cool in the summer; and the space has no other real uses except storage. I’ve already added lighting, moved plumbing, and had an electrician install circuits and a dedicated sub-panel. So I’m committed!

I agree with Hammerthumb and Klutz that involving a professional is the right way to go.

Thanks to all for help with the short and long term fixes to improving my shop! You guys are great.

-- Follow me on Instagram @endgrainy

View CaptainKlutz's profile


4004 posts in 2467 days

#22 posted 07-04-2014 10:35 AM

I want to second Hammerthumb’s advice on using a waterproofing contractor.

Changing a cinder block wall basement into a control living space (or workshop) needs to be done with great care. There is more to it then preventing vapor transmission from the walls, floor, or even the daily humidity. Besides building code requirements, there can be major construction differences required based on your local weather and 100 year flood plane map. Another major challenge is preventing the formation of mold, or finding that an existing remodel is hiding porous walls and not knowing you are growing mold behind the d├ęcor. Calling a couple of basement waterproofing companies (with experience in pan deck ceiling) will help you learn more about any specific local challenges and make sure your space is properly waterproofed.

Best of luck.

-- If it wasn't for bad luck, I wouldn't have no luck at all, - Albert King - Born Under a Bad Sign released 1967

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