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dry film thickness/no of coats etc.

7/7/20       
pascal stgelais

hello

So to calculate dft I use a wet film gauge calculating using the solid by VOLUME.

To get a consistent reading I spray on glass rather than wood. But what has me puzzled is how can a lot of folks here apply multiple coats without breaking the dft.

The manufacturer recommand 4 dft for wich they give an average max of 4 coats sprayed at 3-4 mil. I read here a lot of folk spraying 3 coats( 1 sealer 2 topcoats).

lets says I can spray perfect consistent wet 4mil coat. Even if I spray only 3 coats I will easily break their 4 mil. IRL its hard to be that consistent as some areas will absorb less others much more. Using math I should not spray more than 2 coats including sealer in order to be on the safe side.

On a day to day basis when I spray 2 coats on most wood I get inconsistent sheen and it looks like I have areas that look starved.

ok so now this has me a bit worried that a lot of my jobs (if not all) have all too much build.

if we disregard theory how do you guy handle coats etc. on a day to days basis. I assume you dont mesure your wet film every coats you spray and dont always have a calculator on the side for every coat you spray.

For myself it is generaly selfseal coat...topcoat...topcoat. no calculation other catalysis

7/7/20       #3: dry film thickness/no of coats etc. ...
Mastercabman

Pascal
I use different brand of finish than you
My finish system (ML Campbell)calls for 4-5 mill wet when spraying
But remember that solvent is going to evaporate
Depending on the solid content/volume
The dry thickness is going to be less than what you applied
For example,if you use a product with 50% solids you may end up close to half of thickness than you put on wet
Then you're going to scuff sand your project for the next coat
When Sanding you are removing some of the coating
How much? All depends on how aggressive you are with sanding
MLCampbell states not to exceed 5 mill dry on most of there products
I don't check my wet thickness anymore
I'm just so used to the product that I know how much to put on my project
The only time I check is when I'm using a new product

7/7/20       #4: dry film thickness/no of coats etc. ...
Mastercabman

Also I forgot to add that if you thin your product a lot it will compremised the dry thickness

7/7/20       #5: dry film thickness/no of coats etc. ...
pascal stgelais

same here mastercab I dont measure I usually use the sheen to know where I am.

I mean that when I spray lets say my seal coat I know my sheen will be uneven since some area will soak up the finish while others will already form a film. for example if I spray a piece of fine I need 2 sealcoat to even see that something was sprayed.

Even if you dont measure anymore do you apply the same number of coats or do look at the result to gauge if you are done or not.

for example if I spray birch I know I need at least 3 coats(seal, topcoat,topcoat to get an even sheen. 2. on oak or ash well I am often able to get an even sheen with 2 wet coats(seal, topcoat).

but if I do the math using solid by volume I calculate that I am way over the 4 mil limit if I spray 3 coatsl and on close when I spray 2:

4mil wet 0.36 x 3 = 4.32
4 x 0.36 x 2 = 2.88

and lets say for any reason I overlap just a bit more than 50% (user error)

5mil wet x 0.36 x 3 = 5.4
5 x 0.36 x 2 = 3.6

and even then no toners glaze etc.

IRL how many coats do you guy usually spray. I know this is hard to have a definitive answer since finish is more about dft rather than "coats". But on most of my wood one seal coats and 1 topcoats leave a starved look' rough areas and uneven sheen. Especially since I often grain pop with water
before staining.

7/7/20       #6: dry film thickness/no of coats etc. ...
pascal stgelais

CORRECTION if I spray a piece of *BIRCH* I need 2 sealcoat to even see that something was sprayed.

7/7/20       #7: dry film thickness/no of coats etc. ...
Mastercabman

Ok but don't you sand between coats?
Sanding can take a lot of finish
Especially sealers
When I do a stain job I put 1 coat of sanding/vinyl sealer and 2 coats of top coat
When I paint I put 1 coat of vinyl primer and 2 top coat
BTW going a little over the dry mill thickness they recommend is not the end of the world

7/7/20       #8: dry film thickness/no of coats etc. ...
pascal stgelais

oh my bad

forgot yes I sand using 320 grit between coats for adhesion and also since I water pop the grain before staining and sealing it the firt coat is very rough.

so its more like:

-selfseal CV
-light scuff sand
-topcoat
-serious scuff sand to even sheen and have a smooth base
-topcoat again

I suppose I must be ok since I have jobs over 6 month that did not fail. I also have test pieces that are 1 2 years hold and no signs of failure. I did not thought sanding could take away that much dft

I did forget to take into account the scuff sanding and that I measure spraying onto an non porous surface(glass).

I must also assume that a good part of the sealer is soaked into the wood since there is almost no build/sheen on the seal coat except for very hard dense wood. So its hard to estimate the build thickness vs amount soaked into the wood.

in other word the seal coat must be more like

5 wet mil 0.36 = 1.8 dft - % soaked into bare wood.

This also reminds me of a time where I sprayed a pine table. It took me about 2-3 coats just to get something sandable. I ended up applying about 5 -6 coats if I rember. still going strong after 3 years.

7/7/20       #9: dry film thickness/no of coats etc. ...
DannyB

You can always measure this if it really bugs you and see what's happening for real.

Spray on metal and use a DFT meter to see how thick the coat ends up.

The testers metal that work on metal are like 80-90 bucks and will be good enough.

There are testers that work on wood, but they are much more expensive and complicated, usually like 1500-2000 bucks.

7/8/20       #10: dry film thickness/no of coats etc. ...
pascal stgelais

Thanks danny

I know I am bit OCD but I think I will skip those tester as they are very expensive and in the end we dont measure every time we fire up the spray pump. and good idea on the metal for cheaper alternative but since the cv wont penstrate the metal it might overestimate the real dft

I think that by spraying 1 selfsealing coat and 2 top coats im not overbuilding. although unprecise it seems to be a unwritten rules that I have ead many time here and had followed myself without problem

7/8/20       #11: dry film thickness/no of coats etc. ...
nicko

i used to be really concerned about dry mill thickness too when i first started spraying conversion varnish. I think there is some built in insurance policy that are built in by these finish companies on dry mill thickness. I mostly spray Sherwin Williams CV and i try to spray 3 mills wet and three coats. I use it as a self sealer so my first coat is the sealer coat. I have already run test when i am spraying something i would also spray a 3x3 piece of plywood and double the amount of coats just to see what would happen. i would let these test pieces sit around in my shop for months and never had any problem with them.

Nicko

7/8/20       #12: dry film thickness/no of coats etc. ...
pascal stgelais

thats what I also think nicko I mean if the cv would crack/peel/delaminate as soon as the 4-5mil barrier is broken that would leave no margin of error.

but even then they would probably recommend no more than 1 coat.

I suspect this is more a case of diminishing return. like when you go over 4 mil the toughness does not increase but the risk of failure increase.

7/8/20       #13: dry film thickness/no of coats etc. ...
pascal stgelais

Oh I should also add that I ofen spray on a lazy susan with a construction grade plywood base. that base must receive dozens of thin coats in one project yet this "pile" of CV still did not craze. only one time I left a painter pyramid on it the hole project. The cv literraly glued the pyramid to the plywood. when I managed to pull it out the cv did ot give up...the first layer of plywood did!

7/8/20       #14: dry film thickness/no of coats etc. ...
Scott

I have used ML Campbell, Becker , isf, Ica, and a couple lacquers from Sherwin Williams.
I generally always spray 3 coats max,
1 sealer and 2 top coats, sometimes 2 sealer and 1 top coat. Sometimes just 1 sealer and 1 top coat.
I scuff sand 320 on stain with clear coat and 280 grit on solid color.
Like mentioned I only check material wet thickness on new products.
Different color sheens sounds like you have bad material or your gun is plugging on passes and you are getting a lot of variance in material on your work piece.
I would be scared to spray any more than that, the problems arise 10 fold when you start getting to thick of a dry thickness.

7/8/20       #15: dry film thickness/no of coats etc. ...
Pascal

what I meant by sheen variance is more about how much the cv soaks into the wood on the 1rst-2nd coat.

On very dense wood I get even sheen almost on the first seal coat.

On birch well the wet coat sheen is uniform but when it dries it become uneven since some area are more porous than other (same thing that cause this species to blotch with stain), And get almost no build up

On the 3rd and final coats sheen is uniform. and even then on a walnut table top I had to spray a 4-5 coats on the endgrain since after 3 coats it still had that starved/unven sheen look

Anyway I think you guy all have conforted me that spraying 1 seal coat + 2 topcoats will generally cause no issue in most situation.

7/9/20       #16: dry film thickness/no of coats etc. ...
Scott

Most of your botching comes from your staining process, applying the clear coat may magnify it but you need to get what ever wood you are working with figured out when you stain it.
I like to spray woods that blotch more easily with a toner before I stain them. Different people have different solutions, some people like shellac, some people lime leaving the wood in direct sunlight, you have to experiment with what you like?
Woods like walnut I never clear coat, they will always fade over time, I always apply a natural coloured stain first.
There is really no one way that is perfect for finishing and there are many ways to finish depending on the look and purpose of a particular job.

7/9/20       #17: dry film thickness/no of coats etc. ...
Daniel Shafner  Member

Your coating is closer to 25% solids. Subtract 10% catalyst. Subtract 20% reducer (3.2 x 2). You're now at 25% solids. Spray 3 - 4 wet mils. After drying you have 0.75% - 1.00% dry mils.

So let's say you spray 4 - 5 wet mils. After curing you have 1.00 - 1.25 wet mils.

Take away a little for sanding.

Look at your wet mil thickness gauge. It has teeth. Your applied coating has a thickness that's between one or another adjacent tooth on the wet mil thickness gauge. Hence, the reading, that is to say, the wet mil thickness is always as I've written, between this number and the one number up.

My calculations are based on your previous writings about how much you reduce your coating.

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