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

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Forum topic by endgrainy posted 06-29-2014 07:01 PM 5235 views 0 times favorited 22 replies Add to Favorites Watch
View endgrainy's profile


251 posts in 2809 days

06-29-2014 07:01 PM

Topic tags/keywords: shop ceiling rust slab

Hi Lumberjocks,

I’m having a problem with my shop ceiling. My shop is located in an unfinished room under my garage (connected by a stairway without a door.) It sits adjacent to my basement with a door connecting to the finished part of the basement, so the shop is below grade.

I’ve been woodworking a little over a year and a half, and have accumulated three tools with cast iron tables. Since the shop has no HVAC and thus no climate control, I’ve learned how to deal with rust (thanks to this forum) accumulating from condensation.

A bigger problem has developed this summer with it being so hot and humid I’m the Eastern US. My shop ceiling is made of some sort of corrugated metal that seems to be what they poured the concrete slab from the garage into during construction [please forgive my lack of understand of construction practices and terminology.] The slab is supported by I-beams as well.

What’s started to happen is the metal ceiling is corroding and starting to drip rust in a few spots, falling onto my tools and projects. This seems to have happened at some rate over the 10 years the house has been in existence, but it seems to be worse this year.

I’ve temporarily tried rust remover and spray painting the areas in question, but this is obviously not a long term solution. The ceilings are only about 7 feet high, so not a lot of wiggle room.

I’d like to come up with a longer term solution as I see myself continuing woodworking as a hobby for many years to come. I’d love to one day waterproof the shop and finish it, also with heating/cooling outfitted. But feel I need to deal with the rust issue ASAP.

Some options I thought of:
1) Finishing the concrete garage slab floor (above shop ceiling) with epoxy, thus creating a moisture barrier
2) Sanding the ceiling down to bare metal and applying some sort of barrier against rust
3) Having a contractor come look at removing the metal ceiling – I don’t believe it’s structural, but not sure

I’ll post some pictures to help explain. Any advice is much appreciated!

-- Follow me on Instagram @endgrainy

22 replies so far

View TheWoodenOyster's profile


1335 posts in 2856 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 2809 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


6839 posts in 2641 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 2809 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 3763 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 2809 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 4506 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 2809 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 2466 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


439 posts in 3994 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 2470 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


3163 posts in 3093 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 2856 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


18119 posts in 3927 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 2809 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

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