Shellack under Poly?

[maxwell_smart007]

I have a can of shellack that says not to use it under Poly...I'm making a sign out of bloodwood, and want to seal it before I paint the letters black...

Why can't I use Shellack under poly or spar urethane? Do I even need to seal the bloodwood, or is bleeding not an issue with this species?

Andrew

[rhull]

>I have a can of shellack that says not to use it under
>Poly...I'm making a sign out of bloodwood, and want to seal
>it before I paint the letters black...
>
>Why can't I use Shellack under poly or spar urethane? Do I
>even need to seal the bloodwood, or is bleeding not an issue
>with this species?

It might not be dewaxed shellac.

[Sawduster]

Even if the shellac has not been dewaxed, there shouldn't be a problem with applying an oil based poly over it. The thinner in oil based finishes should also dissolve any wax. Wax in shellac would be a problem with water based finishes.

Now, there could be something else in the premixed shellac used as a preservative or something that might be a problem.

You could seal it with a thinned coat of the poly you're gonna use.

[rhull]

>Now, there could be something else in the premixed shellac
>used as a preservative or something that might be a problem.

THAT'S very true. I didn't even think of that.

[TDHofstetter]

What brand of shellac is it? I'd like to look THAT one up & see what the issue is.

-- Tim --

Member of the
Robert "Limey" Bolton Memorial
International
Volunteer Mentorship and Assistance
Programme

[maxwell_smart007]

It's a can of Zissner Shellac...I was going to topcoat it with Varathane Diamond Spar Urethane...

I've routed out some letters in a sign made of bloodwood, and was planning on painting the routed letters black...

http://www.zinsser.com/product_detail.asp?ProductID=31

Does it matter what kind of paint I use too?

Andrew

Thinned poly, eh? Would I just thin it with Mineral Spirits? Half-and-half?

Andrew

[maxwell_smart007]

I rubbed it down with a very little BLO as well (everywhere that wasn't routed), and I'm waiting for that to fully dry before I put anything else on it...

Andrew

[rrich]

Andrew,
I heard something today that may apply, but you be the judge. A bit of work was sealed with shellac. Then a poly was sprayed over it. The results were orange peels.

Was the shellac de-waxed? I don't know.
Was the spray too thick? I don't know that either.
Was the person who did the spraying competent? I don't know that either.

My suggestion is this. Take a piece of scrap and sand it the same as your project. Put the shellac on the scrap and let it dry for a day or two. Then wipe on some poly. Don't do anything special just wipe the poly on.

Then the question is, do you like the results?

[TDHofstetter]

I'm completely stunned - I've got the TDS for it in front of me, and it reads "Bulls Eye Shellac is not recommended under polyurethane
finishes". I can't fathom why - polyurethane thins with mineral spirits while shellac thins with alcohol, so there can't be a solvent issue. I also can't imagine that anything in the poly would attack the shellac. The only thing I can think of relates to the next thing it says: "To seal wood before finishing with polyurethane use Bulls Eye® SealCoat™ Universal Sanding Sealer"... which indicates to me that they're in the market to sell stuff & I'll bet nearly anything the SealCoat sells for more money (they're drumming up more business).

I don't think I'd hesitate to use poly over shellac any day of the week.

Nope, kind of paint shouldn't matter, either - as long as you use a paint that can handle the target environment for this sign.

You could definitely use thinned polyurethane as a primer coat; very VERY often a thinned coat of the final finish material is used as a primer coat - the point is just to get it to bind better, which happens with thinned material... soaks into the wood deeper than normal finish.

If it's water-based poly, thin with water. If it's conventional oil-based poly, thin with mineral spirits. Anything in the range of 1:3 (poly:thinner) to 1:1 will work for primers, depending on the porosity of the wood. For dense woods with little porosity, I'd go with a VERY thin mix - for coarser woods with great porosity, go for the thicker mix. For end grain, straight unthinned poly will often do entirely well enough. Since you're working bloodwood (gods, I like that stuff!), I think I'd go with something around a 1:2 mix - that's pretty thin, 'cause bloodwood is pretty dense.

-- Tim --

Member of the
Robert "Limey" Bolton Memorial
International
Volunteer Mentorship and Assistance
Programme

[dbriski]

The shellac is not dewaxed and I think that is why they are saying not to put the poly on. The Seal-coat version is dewaxed. Also the shellac comes in 3lb cut and the seal-coat in 2lb cut. I think the prices (per gallon) are fairly close with seal-coat marginally cheaper. So I don't think they really make that much more money between the two different products. Seal-coat is less shellac but also requires more processing to remove the wax.