How many coats is too many?

I did my own stairs…..Red Oak. I used an oil based polyurethane. I prefinished the stirs before I installed them. I did 6 coats. Doing stairs in place in an occupied house is not fun so I did the 6 coats of a high quality poly.

If the finish is thicker than the lumber it is time to stop.

I did my own stairs…..Red Oak. I used an oil based polyurethane. I prefinished the stirs before I installed them. I did 6 coats. Doing stairs in place in an occupied house is not fun so I did the 6 coats of a high quality poly.

- JIMMIEM

Same. I bought unfinished red oak re-treads, which I’m finishing before install. The Waterlox Urethane is oil based (soybean oil). I’m currently at 4 coats, and was going to just keep going until the can ran out. At the rate it’s been going though, that could be 8 coats. Seeing as stairs are the most high traffic place in the house, I don’t think 8 coats would be a bad thing. But I wonder if it’s a waste.

-- ~Walker

If the finish is thicker than the lumber it is time to stop.

- Kazooman

Good one Kman!

-- Aj

Interesting topic, thought everyone knew this information? Must only be us chemistry nerds? :-0)

First: How thick is each coating? hehe

Lacquer has recommended maximum dry film build of ~0.004” thickness. Hybrid nitro cellulose acrylic systems usually recommend 0.005” max. If you go much thicker, the coating gets too rigid and will crack/craze due normal wood movement due temperature and humidity.

Varnish blends are based on more flexible ‘hard’ resins than lacquer, but still recommend 0.005 to 0.006” as max thickness. They tend to be more tolerant than with lacquer, when to thick.

Polyurethane finishes are the most flexible of wood coatings. But due to larger polymer chain length tend to allow more moisture vapor permeation than hard resin coatings. Though they are better at stopping alcohol, and other solvents from reaching the wood. So like everything else in finishing, there are trade offs. Polyurethane can put down over 0.020” thick if needed for special reason (bar top?), but rarely need more than 0.006 to 0.008”

Generally speaking once exceed ~0.008” (just below 2 sheets of 20lb copy paper) film thickness of a coating you have reached the maximum level of environmental protection the polymer system can provide. Many polymers reach 80-90% protection with only a couple thousandths, so there are diminishing returns with more thickness.

Film thickness can be double edge sword:
One extreme example – silicone rubber water vapor permeation can benefit from a heavy 0.025-0.030” film, which also provides more physical protection from impacts, just like a rubber bumper. At same time, a thick film of Polyurethane is more easily damaged via impact, due the thick mass of ‘softer than wood’ on surface; which means you can dent the poly and not the wood.

BTW – just because you have finish on wood, does not mean moisture in wood doesn’t change. All the coating does is slow the permeation of the moisture. Some films like silicone rubber have very high transfer rates, which is why you don’t see silicone coatings for wood. Polyurethane has higher permeation rate than hard resin coatings, which is one reason most manufactures suggest a thicker ‘plastic like’ film .vs. the barely visible film used with lacquer, or oxidizing (air hardening) oil finish.

Hope these summary helps?

#IAMAKLUTZ, not an expert. Been retired awhile and some days I forget more than I remember. Any experts out there that see errors above, please correct the mistakes, and skip the flame job. Already know I am Klutz.

YMMV

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

I did my own stairs…..Red Oak. I used an oil based polyurethane. I prefinished the stirs before I installed them. I did 6 coats. Doing stairs in place in an occupied house is not fun so I did the 6 coats of a high quality poly.

- JIMMIEM

Same. I bought unfinished red oak re-treads, which I m finishing before install. The Waterlox Urethane is oil based (soybean oil). I m currently at 4 coats, and was going to just keep going until the can ran out. At the rate it s been going though, that could be 8 coats. Seeing as stairs are the most high traffic place in the house, I don t think 8 coats would be a bad thing. But I wonder if it s a waste.

- Walker

The thicker the poly finish the longer it will last. I confess, I really did 7 coats. The thicker it is the longer it will take for the foot traffic abrasion to wear it out. While on the subject, depending on your footwear, e.g. socks or smooth bottom footwear, the poly will be slippery. Might want to consider installing a stair runner carpet.

If the finish is thicker than the lumber it is time to stop.

- Kazooman

Thats not a coat, thats a parka.

OP, it depends what kind of finish. BLO, I use 3 max. Some like Polyx say use 2 or 3, etc.

CaptainKlutz you always have such thorough replies full of science and stats! I’ve appreciated many of them. I don’t have anything that could measure thickness that accurately, but I do have two sheets of copy paper to compare to. I wish I knew approximately how much thickness each coat added. Of course, I recognize now that instead of posing the question as number of coats I should have referred to total thickness.

Lacquer has recommended maximum dry film build of ~0.004” thickness. Hybrid nitro cellulose acrylic systems usually recommend 0.005” max. If you go much thicker, the coating gets too rigid and will crack/craze due normal wood movement due temperature and humidity. a thick film of Polyurethane is more easily damaged via impact, due the thick mass of softer than wood on surface; which means you can dent the poly and not the wood.

- CaptainKlutz

These are two examples in which too much thickness is a detriment. Which is exactly what I was curious about.

The thing that’s making me unsure is that the can says it has approx. 150 sq ft coverage, and recommends 3 coats. My project is 50 sq ft. So 3 coats should use the whole can. However, after 4 coats I’ve used only half of the can. I do try to apply thin coats, but I don’t feel like they’re going on that thin.

For the record, on smaller projects I typically use two coats of danish oil followed by 1 or 2 coats of poly and find that to be sufficient. With shellac, no matter how many coats I feel like it’s not enough.

-- ~Walker

If the finish is thicker than the lumber it is time to stop.

- Kazooman

Awesome

-- Think safe, be safe

I don t have anything that could measure thickness that accurately, but I do have two sheets of copy paper to compare to. I wish I knew approximately how much thickness each coat added. Of course, I recognize now that instead of posing the question as number of coats I should have referred to total thickness.
- Walker

Just happy to help pass along knowledge I’ve learned as younger man.
Here is additional info that might help?

Regarding film thickness:
Normal method for measuring the film thickness of wet coating is done with a ‘notch’ gage.
Reference an expensive and complicate national standard >> https://www.astm.org/Standards/D4414.htm
But seriously…..

There are many different types of notch gages.
Gardco lists most of the common configurations towards the bottom on this website page:
https://www.gardco.com/filmthickness.cfm

The most common for painting application comes in this shape:

or this shape:

But there are round versions for painting curved surfaces.

Most industrial finish suppliers carry plastic notch gages with private labeling on them as sales & marketing tool. Often see them on order/will call counter, sometimes labeled for couple bucks each. If you buy a decent amount of stuff usually find one tossed in your box as freebie just for asking how much one cost, along with pile of mix sticks, or disposable paper filters. They are sort of like that free antique car magnet Rockauto throws in every box? lol If nothing else, Amazon and some Auto parts stores have them.

The challenge with wet film thickness is knowing the solids content on your coating. Then you calculate the approximate reduction due drying process. Most industrial spray coating mfg share this information on data sheets, it is only the retail or hobby grade finishes that make us guess.

There are really simple to use. Place the side into your wet coating, and then lift and look which pins (between the notches) is wet and which is dry. You thickness is between the two. Super complicated, guess you don’t really need the standard. Haha

You can control the wet film by viscosity of material and application method. There are limitations in how thick you apply on vertical surface .vs. horizontal surface without runs or drips; which further complicates the finishing process.
IMHO – if retail marketing folks told everyone just how complicated wood finishing can be, many would pick a different hobby. That is why wipe on poly is very popular at BORG. ;-)
BTW, if you wipe on your finish, be surprised if you could measure the thickness. and that is another reason I didn’t talk how how many coats is enough earlier – not all coating methods or materials create same thickness.

Thanks for reading my gibberish. Hope this helps!

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

Captain, I think you have covered the subject with a generous layer of knowledge, but in my case it’s running and dripping… (metaphoric mumbling from me..)

I’m an engineer, but I’m tired of thinking.

Still, this is exactly the kind of real knowledge that ought to be passed along here.

People who didn’t ask a question still get answers.

Thanks
-Paul

-- I intended to be a woodworker, but turned into a tool and lumber collector.

Captain, I think you have covered the subject with a generous layer of knowledge, but in my case it’s running and dripping… (metaphoric mumbling from me..)

I’m an engineer, but I’m tired of thinking.

Still, this is exactly the kind of real knowledge that ought to be passed along here.

People who didn’t ask a question still get answers.

Thanks
-Paul

-- I intended to be a woodworker, but turned into a tool and lumber collector.

With nitrocellulose lacquer I never count or measure. 8 – 12 coats doesn’t matter.

With pre-cat lacquer I have done so much of it I don’t measure any more.
With my material and mixture I can spray 4 – 5 coats and be within the 4 mill spec.
(Edit: But 3 coats is usually enough)

When I spray conversion Varnish I measure a control sample before I start spraying,
and then after I spray 2 coats and it’s dry I measure it with a micrometer. Then I decide how many more coats
it needs or I can apply without going over 4 mills.

For floors, I’m not in the camp that more is better.

On Amazon there are “notch gauges for painting” that measure film thickness electronically, $119 up. News to me, don’t plan on doing that. Use the number of coats that has worked for me in the past. Did like and appreciate CaptainKlutz information and knowledge.

-- It's not a mistake it's a design opportunity

Too many coats and your going to have a soft finish. Hardwood floors 2 coats, my furniture 2 unless there’s a problem and that’s with pre-cats…I can spray 1 heavy coat equal too 2 coats. Factories have a system but individuals dont..

I was on a body shop forum….the guy said I painted my car with 13 coats of lacquer and I have a rust spot. What do I do? Main dupont guy said fix it and repaint it 13 times… guy was pissed…