Decoupage? I think lacquer is used to coat a decoupage and the wood
is prepped, somehow, prior to the application. For the very thin
paper used in most decoupage, there is a particular technique for
applying it. I don't recall the specific name of the thin paper, but
it's similar to the light paper used in packaging delicate gifts, more
often womens' gifts. Tissue paper of some kind. Maybe do a search
for decoupage applicating.
Otherwise, a decal, as I understand them, is a thick paper tape
application. I would suppose lacquer would be appropriate for coating
that, also, as long as the decal doesn't have some sort of wax finish/
coating on the decal face, itself.
If there is much as stake here, do a "practice run" with a decal similar
to the one you plan to use and a piece of wood you don't care about.
Then after a suitable amount of time, see what it takes to scratch the
decal off. I've seen decals hold up quite well without any additional
protection. Everything just depends, depends, depends...
I've done it years ago using urethane varnish (varathane) to cover and
seal. The wood was varnished before aplying the decal, and after.
I've also seen them done with shellac, but that has it's own
vulnerabilities, so it depends what you are putting it on.
Are you meaning a real "Decal" which is called a Decalcomania, which is
nothing more than a paint film that you slide off gummed paper after wetting
it. It will only stick to smooth non-porous surfaces not bare wood. It can
be sealed but some lacquers and varnishes may damage the decal.
Or are you meaning self-adhesive label, that some people call a decal, that
you stick onto the surface. Bare wood will dry out the adhesive causing the
label to eventually drop off. It should also be stuck to a non-porous
surface. Coating may damage the inks, vinyl, paper or adhesive.
Decoupage, sometimes confused with a decal, is covered by another reply.
I have mounted the above decals to wood after sealing. I have made
propellers for awards and mounted the likeness of the decals from
propeller manufacturers on it.Seal it with dewaxed shellac, or lacquer
I don't know how they survived as I lost touch with that group of people.
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