Fisheyes can appear in a waterborne finish if the wood's surface is contaminated. Seal with dewaxed shellac first.
Problem #2: Waterborne finishes tend to raise the grain. They can also develop "fisheyes"—small round craters caused by oil, wax, grease or other contaminants in the wood.
Sealer Solution: Dewaxed shellac. Seal the surface with a coat of Zinsser SealCoat to limit grain raising, lock in contaminants and make the waterborne top coat behave as it should.
Problem #3: Stain and clear topcoats won't cooperate. Either the stain bleeds into the topcoats or the topcoats don't adhere to the stain. For example, water-based stains often bleed into water-based topcoats, and sometimes water-based topcoats don't adhere to oil-based stains.
Sealer Solution: Separate the stain and topcoats with a barrier coat of dewaxed shellac. A thin coat of 2-pound-cut dewaxed shellac will seal in the stain and provide a base that both solvent-based and water-based coatings will stick to.
You can make your own dewaxed shellac by dissolving dewaxed shellac flakes in alcohol at the ratio of 2 pounds per gallon, or you can buy Zinsser SealCoat. It's liquid dewaxed shellac in a 2-pound cut.
Problem #4: Polyurethane and lacquer finishes are hard to sand, so when they're used as the sealer coat, they take a lot of effort to level out.
Sealer Solution: Sanding sealer. Topcoat finishes are meant to resist scratches, and that goes for sandpaper scratches, too Sanding sealers are formulated to dry quickly to a (relatively) thick film that sands easily—perfect qualities for an initial finish coat. Sanding sealers are not suitable for use as topcoats, unless you want a finish that scratches easily.
Problem #5: Disastrous finish results with oily woods, such as rosewood and teak. Rosewood is notorious for preventing varnish from curing, and its dark color often bleeds into lacquer and discolors adjacent light-colored wood. Some oily woods let finishes cure, but the dried finish peels off.
Sealer Solution: Zinsser SealCoat (liquid dewaxed shellac) is the best choice for home shops. Commercial shops often use vinyl sealer. Both will adhere to and seal oily woods.
Vinyl sealer is available from paint dealers. It has to be sprayed on and it requires spray booth ventilation. If your topcoat is lacquer, spray on a coat of vinyl sealer and follow with the first coat of lacquer no more than 45 minutes later, before the vinyl is completely dry (lacquer adheres poorly to fully-cured vinyl). If the topcoat is varnish, let the vinyl sealer dry overnight, but scuff it with an abrasive pad before you apply the varnish
Water-based stains and dyes can bleed into waterborne finishes. Sealing the colored surface with dewaxed shellac before applying the finish is the solution.
Sanding sealer is much easier to sand than polyurethane, and its sanded surface allows topcoats to firmly adhere.
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