Many products say that the topcoat is self sealing, but they also sell them alongside a vinyl sealer. How come? Is a vinyl sealer better than using the topcoat as its own sealer? It sure is easier using a single product, but is quality compromised?
From contributor R:
Many coatings can be used as a sealer/topcoat - kind of an all in one product. My guess as to why a sealer is also available is because that sealer can be used under another type of coating, in other words a non catalyzed coating or an oil based varnish and in some cases an oil based pigmented paint.
Sealers can also be easier to sand than the all in one coatings. That said, some finishers like to apply the sealer over a stain or a glaze and because they are a bit easier to sand, it's less likely to burn through.
The sealer alone can be used as a barrier coat when re-coating an existing finish. Some claim that the KCMA approves the vinyl sealer under the topcoating due to it being a good moisture barrier. There are pros/cons to this assumption. It really depends on what you're going to be finishing/refinishing as to whether to use sealer or not.
In this case when a non cross linking finish like nitro lacquer is the topcoat, the vinyl does in fact provide better moisture resistance than nitro lacquer could if used as a self seal system. It gets sketchier when using pre or post catalyzed finishes. True, some vinyls sand better than some topcoats, but others do not. And not all pre cats are created equal when it comes to moisture resistance. Some are not much better than nitro lacquer and could benefit from the use of a vinyl sealer. Other pre cats are almost as resistant as post cat finishes and do very well when self sealed.
Other uses for vinyl (as others have stated) include a barrier coat when using products that normally may not like each other, such as solvent based stains and water based topcoats. There are also some glaze products from certain manufacturers that recommend sandwiching the glaze between coats of vinyl. Beyond this there is not much reason to use vinyl sealers if the topcoat you are spraying has good moisture resistance and scuff sanding properties. But some still believe (party due to marketing hype and partly from the old nitro and cab lacquer days), that a separate sealer is necessary to "seal in" the wood before topcoating. And finish distributors are very happy to sell these folks all the vinyl they want.