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dry film thickness/no of coats etc.7/7
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
Also I forgot to add that if you thin your product a lot it will compremised the dry thickness
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
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
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
CORRECTION if I spray a piece of *BIRCH* I need 2 sealcoat to even see that something was sprayed.
Ok but don't you sand between coats?
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:
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.
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.
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
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.
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.
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!
I have used ML Campbell, Becker , isf, Ica, and a couple lacquers from Sherwin Williams.
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.
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.
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.