With MDF you can seal the MDF a few ways.
Mix glue and water (waterbased glues only) and cover the cabinet,seams really well.A few coats may go into the seams.
But the glue is on hand.
Some people use Shellac as an option
Now if you use the glue for veneer from Joe Woodworker.com this glue is very thick and brush on one coat on all the ends of the MDF.
Then roll out a single coat over the cabinet.And one coat on the veneer and you are good to go.
With the binders in this glue it is very thick and seals MDF in 2 coats at the most.
Now if you use solvent based contact.
You can use the same method as this glue from Joe Woodworker.com.
Let the contact dry on both the cabinet and veneer,100% dry let it sit for days,months does not matter.
But when you want to apply the veneer just position the veneer on the cabinet and iron it on.
No need to worry about it sticking before you line up the veneer to the cabinet.
Nice job on them AV3
