This is another really common problem with polyurethanes and acrylics. If your piece yellows within a week or so of application, this is actually not yellowing. It is a result of leaching.
Porous materials like brick, wood, and paint contain chemicals that can leach into your clear coat and yellow it. This is not due to the clear coat itself; it is due to the makeup of whatever you’ve sealed.
Some of the chemicals that can leach are naturally present in porous surfaces. Other chemicals accumulate there over time. For example, if you thrift a chair that was used in a smoking lounge, it is almost guaranteed that the chemicals from the smoke are now present and will leach into your polyurethane.
Again, this is not the fault of the poly you purchased. Some chemicals will be pulled from porous surfaces no matter what you do.
Solution: ALWAYS test your polyurethane on a small, inconspicuous corner of your project. Let it sit for a week and check for leeching before painting the rest of your project.
If you’ve got a fully sealed piece on your hands that is suffering from leaching, the good news is that the leached chemicals are now sealed in the polyurethane. You should be able to scuff-sand the piece, repaint, and seal without any new leaching.
