If a finish is already applied, sealant is unnecessary.

The sealant is necessary because para wood is actually wood from a rubber tree and contains traces of latex.

The gotcha with latex is it makes most finishes difficult to apply.

However, should you insist on adding another finish, shellac will adhere----but, shellac needs to be fresh(less than three years old in liquid form).

Not sure what you read or what they recommend as a "sealant."

A film-forming finish (i.e., anything other than oil or wax) will seal the surface. Why is there a need to add more? If you plan to add more, it's helpful to know what's on there now. Most finishes will take a compatible top coat; some will not.

Just shooting in the dark you are as likely to do as much harm as good. I am reminded of a customer who bought a new dining set and wanted it a little "shinier." She applied a thick layer of polyurethane that within a few weeks had peeled off about 20% of the surface and the rest looked just terrible. it was a lot of work to strip it all off and refinish to accomplish what could have happened with a few minutes of buffing.

That said, parawood is often used on the low-end furniture so the finish may not be the most durable.

The half-right rule: "Half of what you read or hear about wood finishing is right. You just don't know which half." -- Bob Flexner