If I used dewaxed shellac as a primer/seal coat and then followed that with a waterbased poly would I have any adhesion problem?. My concern stems from hearing that water and shellac do not get along well. My thinking in using the shellac first is to reduce the amount that the grain would be raised when putting on the water based finish. Any advice would be appreciated!
Geoff
Replies
Shouldn't be a problem
I have used dewaxed shellac as a seal coat before applying Minwax Polycrylic without a problem. I did it for the same reasons; lessen raising the grain. However, I was disappointed in the amount that the grain still raised. The denatured alcohol solvent for the shellac has some percentage of water depending on the percent of alcohol. Regular denatured alcohol has a rather high moisture content. You can buy almost 100% alcohol but once you open the can and start using it, it starts absorbing moisture.
Shellac and Poly
Try using a water based shellac instead of dewaxed shellac. yes they make it. You can get it from a company called Target Coatings (not related to Targets stores). I'm using one called UltraSeal-WB Shellac, Amber. I t will be used on African Mahogany in a two seal coat and Amber color to give it a warm color. Then four coats of water based polyurethane from General Finishes, High Performance Water Based Top Coat. Satin. Before any of this I am going to try a new product called Charles Neil's Pre-Color Conditioner. it will stop blotching etc. Everything is water based.
http://www.targetcoatings .com
http://www.generalfinishes.com
African Mahogany is a wood that colors evenly when a stain is applied. No pre-stain treatments are necessary. Applying one will only reduce the intensity coloring of any stain subsequently applied. Test it out on some scrap before you commit to your finishing schedule.
Using waterborne shellac--ie. it's dissolved in an alkali solution not in alcohol isn't automatically going to do anything to make it better under waterborne finishes, though it may have chemistry designed to be compatible with Target coatings.
By the way, shellac is quite water resistant, certainly capable of handing water in a finish for the time it takes to evaporate. Sure if you leave water on it overnight it could have a problem, but waterborne paints dry WAY faster than that.
You won't have any adhesion problems if you use a dewaxed shellac under a waterborne finish.
If your first coat will be alcohol based dewaxed shellac you can sand the wood to 400 grit, which may greatly reduce grain raising. Otherwise you probably need to stop at 240 grit (see instructions for your wb finish) but can "raise the grain" before applying finish. After your final sanding, lightly wet the wood. When dry, lightly sand with the same grit. There likely will be less grain raising then from your first coat of wb finish. Regardless, thoroughly test the complete finishing schedule on equivalent scrap before beginning on the finished piece.
Personally I prefer to sand raised grain after the first coat of finish, be it shellac or the waterborne acyrlic. The finish hardens the raised fibers and makes it easier for sand paper to slice them off. It's very hard to sand exactly the right amount after just raising the grain. Sand just a little too much and you will have released new fibers to raise when a finish is applied.
Grain Raising
I agree Steve,
Here is a excellent article on just that point.
http://www.hardwoodlumberandmore.com/Articles/ArticleViewPage/tabid/75/ArticleId/28/Pre-Raising-The-Grain.aspx
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