Shellac and Poly

[gabacus]

Hi Guys,

i was reading an article on finishes and they claimed you should not use poly on top of shellac, but they dont say why. is this true? i am working on a project at the moment and i was planning to use white shellac as a pore filler and then go over it with some poly... what will happen if i do this?

cheers,

gabacus

[durwood]

Shellac only dissolves in metho, it makes a good finish by itself as it can be put on in several coats and worked (french polished ) to fill and level into a smooth glossy surface.

It is often used to seal off previous coats of material as the metho attacks very few if any other finishes and it stops the next coats from soaking into the timber.

It will therefore dry and when you put another type of finish on top again it's solvent won't react with the old finish as its sealed off by the shellac the new paints solvents won't hurt the shellac as it is only dissolved with metho. So the new finish can be safely applied to produce a different new surface.

Two problems with using the shellac.
1. it doesn't like heat, so if you put a good heat resisting finish like poly on top the shellac will be affected and the poly will fail.

2.the new and old finish won't be affected by the metho in the shellac which means the shellac just dries in the middle without sticking well to either surface. It can therefore lead to poor adhesion and the shellac may let go from the old finish or the new finish may not stick well to the shellac.

Depending on what products are involved will depend on what happens but using shellac under such curcumstances is alway regarded as a quick fix and not used as the prefered method of finishing.

Paint sealers don't use shellac but they are based on metho as the solvent, the second rule applies here also they fix the reaction problem but you loose some of the adhesive properties of the finish and risk "peeling" especially if you test the adhesion out by using something like masking/stickie tape on the surface and then pull it off.

It could also fail if you rubbed the new surface back (to fix a scratch etc) for instance and you break through to the sealer it may be difficult to get edge to stay down, the paint tends to peel off the sealer.

[gabacus]

Thanks for the durwood! i understand what you are saying!

I asume that it is still okay to use a shellac based sanding sealer (the u-beaut product) before using poly? or have i completely missed the point...