On Mar 19, 9:08*am, Swingman wrote:
On 3/18/2013 6:48 PM, SonomaProducts.com wrote:

You're done.

After the second coat you have a finish that will last a lifetime unless it is a bowling ally or highly used table top or other high traffic surface.

Not necessarily.

Not necessarily, indeed.

"Thinning", which he admitted to doing by approximately 50%, simply
means the layers of poly he applied were 50% thinner, meaning more
layers/coats are need to be applied to reach the same level of
protection as the unthinned product.

Important to remember here is the method of application. I read this
over and over here and this is what I get from it. "I flooded the
surface and wiped off all the finish I could until it looked smooth."
Using jersey cloth you can easily get the finish left behind to 1
mil. of thickness. Using the average MSDS data, when the carriers and
solvents leave the unthinned product, the coating will shrink in
thickness about 66%.

So if one has thinned it further on their own, they will get 1/2 (from
thinning at the shop) the amount of 2/3rds thickness less (remaining
thickness after normal drying of unthinned product), or 1/6th the
amount of thickness recommended by the manufacturer.

You now have what is known as a "dust coat". Further application of
thinned product in the same manner will increase the thickness only
marginally, so one may now have as much as 1/3 the amount of finish
recommended by a manufacturer. But really, do you?

Probably not. The initial coating of finish will get more "traction"
and will more readily adhere to the surface. However, the second coat
will face a smoother, almost sealed surface and your application
method of applying thin coats with a rag can leave as little as .5 mil
coating behind. Figure in the shrinkage of the thickness after the
solvents and carriers are gone...

When contracting for a polyurethane hardwood floor finish I spec four
coats when "thinning" up to a maximum of 25%.

I would submit to you that it is not an apples to apple comparison.
In your situation, in my opinion, yours is exactly what is needed.
Their are two huge differences. First, your flooring guy doesn't put
down a few gallons of finish, then attempt to wipe off most of it.
Flooring urethanes are formulated different, and without thinning they
are almost impossible to get the correct layout of the product. They
are made to be thinned, probably to the 20% range. (NO ONE could
detect a 5% differential... probably not even a lab.)

Second, in an exactly controlled application as you speak, if you thin
all coats 25%, but put on 4 coats the manufacturers application
thickness requirement, you are exactly where you need to be when
finished. Perfect.

In his situation about the only benefit of thinning the poly is faster
drying.

Absolutely true. Thin coats of product do not provide adequate UV
protection (think down lights, tables by windows, etc.) nor do they
provide more than the smallest amount of abrasion resistance. These
are finishes for extremely light use such as in the homes of older
adults with no kids (and no drinking or eating buddies) and for pieces
to be admired. Easy to put on, nice to look at, but little utility
value.

Robert