Fish scale like reaction of poly-urethane overs dewaxed shellac

[Martin-IT]

Hi,

using dewaxed shellac with a few coats of poly-urethane has been my goto solution to finish cherry for a few years.

I am finishing a piece which had 2 sides with 1/4" cherry plywood.

For once, I thought of finishing the plywood prior to set it in the piece, with the 2 coats of shellac, planning to put the poly once assembled.

Now, all the pieces has had 2 coats of dewaxed shellac, every is finishing normally, except the 3 pieces of plywood.

The poly is cracking like 'milk-paint' does, but only on the plywood.

I may have used an older can of dewaxed shellac for the plywood vs the rest of the piece, but I am not sure.

I have sanded it, clean it with denatured alcool, recoat it, and the same thing happen again.

So, do I sand it, put a coat of the fresh shellac on (will it stick if some poly is left ?) and try putting poly on again ?

I once did the common mistake of sanding thru the veneer, so I am trying to stay away of resanding the plywood...and it is now in the piece of furniture.

Any advice ?

The good news, the top of the pieces is not having this reaction, not being made of plywood.

 

 

[Ronn W]

My only thoughts are questions:  1) Did you let the shellac dry completely before coating with poly? 2)  Are you using thin coats of poly (not to thick) and allowing evenough time between coats?   Other than those thoughts I will leave it to more experience members to brainstorm this one for you. Ggod luck.

[drzaius]

Generally speaking, cracking happens when a faster drying finish of one type is applied over a slower drying, or not completely cured coat of another type of finish. My thinking is that the old shellac was not completely hardened & then the poly was applied. The film of poly wants to shrink a bit as it dries & cracks appear cause the soft shellac can't hold it in place & allows a bit of movement.

The veneer on plywood will absorb more finish than solid wood. Maybe it absorbed enough that it takes that much longer to fully dry than the solid pieces.

[Martin-IT]

The shellac on the plywood was applied a few weeks back. It is at least a year old, and may be it is past its usable time.

So, trying a coat of the newer shellac on top ? at this point it cannot make it worse.I will try on the smallest section tonight and see.  Even the top coat, is a few days old.

Either there was some finish on the plywood ? I bought it from the supplier a few years back. I just lightly sand it, as I got burned in the past going thru the veneer.

[drzaius]

I'd try washing the old off with alcohol & a plastic scrubbing pad, then a coat of fresh shellac. I've seen old shellac that stayed soft for weeks after application.

[Martin-IT]

what i did:

-send the pieces with 400 grit

-clean it up, using plenty of denatured alcohol 

-wait for it to dry

-2 more coats of fresh dewaxed shellac

-sanding after with 400 grit

-recoats with 2 coats of varathane floor finish. I used in the past the old formula of the same finish. The new formula is almost as thick as oil finish and does cover in only 2-3 coats vs 4-6 coats with the previous formula. I tested the old formula against alcohol, water, acetone and it did not affect the finish (old urban legend regarding water based finish that they did not resist). I did not do the same chemical test with the new formula.

At this point, where I could sand well the plywood, the refinished plywood is not having the fish scale. 

My theory: the old dewaxed shellac is either past its shelf life, or I may have been contaminated last time I used it with something.