| Thanks. If it's for water based paints, it wont work on polyurethane.
|

No, but sometimes people refer to things like
Varathane as water-base polyurethane. Though
that stuff dries very smoothly. For that reason
I was hesitant to answer the OP. It sounds like
they don't know what they're asking about and/or
couldn't be bothered to explain it.

| Many years ago, I brush painted a truck with Rustoleum (oil base) and
| used some similar product but it was made for oil based paints. It
| worked quite well. I think it was called Penetrol. Dont know if they
| still sell it, or if it would work on poly.... The best advice, "If in
| doubt, call the manufacturer".
|

They still sell it. I used it for years with interior oil
trim paint. It was great. But it was radically changed
with the move to accomodate EPA fume regulations
and is no longer the same product. I haven't tried
the new version. I doubt it's any good. I've yet to
see an oil paint that meets the EPA standards that
isn't junk. So I'm guessing that Penetrol is a similar
case. Only some brands in quarts, which are exempt,
are worth using. I think Rustoleum is still good, but
I've never actually used it very much. Sherwin Williams
quarts are the only brand I currently know of that I'm
sure are still good. (Benjamin Moore downgraded their
whole line so that they could conform while still selling
gallons. As a result, they no longer make any good oil
paints. SW took the approach of eliminating gallons
in order to keep making the same good paint. Pratt
and Lambert did the same. But they've now been
bought by SW and seem to be disappearing.)

I once brush-painted my own pickup with One Shot
sign lettering enamel. Beautiful stuff that dried perfectly
smooth and stayed tough. It was in a class of its own;
a specialized commercial paint. But that, also, got
downgraded.