Varnish is sometimes diluted for the first coat for better adhesion to
porous surfaces. Not really a question of penetration but yes, it probably
is good practice but not really for penetration and not exactly to form a
The last coat is sometimes diluted too, to get a better finish, like dadioh
says it is easier to brush out.
Brother has rv and there is moisture in a small area of one of the walls.
Really hard to replace wood but have removed wood panel and was thinking of
replacing it with 1/4 cdx with min wax spar urethane .
Since this is in an RV, I'd use 1/4 A/C ply with all surfaces,
including edges, sealed with 2-3 coats of laminating epoxy.
(Doubt anybody stocks anything but A/C in 1/4)
Since this is inside away from the sun the epoxy won't be exposed to
the sun's UV rays so no need to protect epoxy from sun.
Sand with 100 grit after 48 hours cure time to prep for next coat of
Buy a dozen, 2" chip brushes on sale @ HF, then use one time and throw
Costs more to clean them than they are worth which is about $0.25
The RV will be in the dump and those epoxy coated panels will still be
This is typical for professional finishers in the wooden boat business, for
initial coats. The first coat would be diluted 50% (the pros I know use
turpentine, not mineral spirits), and the second coat would be diluted 25%.
After that it's full-strength coats for most spar varnishes, except for the
Epifanes product which is too thick as-shipped for brushing. For that
product they continue to reduce it with a little turps and maybe a splash of
Penetrol for better brushing and wet edge retention.
We tend to think that we get better penetration by doing this, and it's
probably true for soft woods, but teak - used a lot for exterior woodwork on
boats - is so oily that I doubt if it penetrates very much. It does make a
nice base for the later coats, though. It would be interesting to do it on a
scrap piece and then cross-cut it when it's dried and look at it with a
loupe to see if you could actually identify any visible penetration.
If you're counting coats, we count the first two thinned coats as just one.
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