This article was inspired by a question from Jeremy. He writes:
I am about to get started on a set of bookcases made out of MDF. I propose to paint the bookcases white. I will probably use a nitrocellulose lacquer and apply it with a spray gun to get a high gloss finish. The last time I did something like this, I did the whole glue up first and then spray-painted the assembled piece. That worked okay but it meant that there was some tricky finishing of the inside surfaces and it was difficult to avoid runs on the back and inside faces. This time I was thinking of pre-finishing the uprights, shelves and back before assembly. I would do one final finish coat after assembly to hide any stuff ups!
Should I pre-finish the pieces when I have them cut to the final sizes but before routing the rebates and dadoes or rout the rebates and dadoes and then do the pre-finishing? If you think the dadoes and rebates should be cut first, I suppose I should mask them when spraying so that the lacquer does not get in the way of a strong glue joint when I come to assembly. Or could I just glue up the painted pieces using epoxy? If you think the pre-finishing should happen first, should I mask where I am going to rout the rebates and dadoes or could I do the routing through the pre-finish?
I confronted this issue just recently with the steamer trunk project. You can view the video here if you want to get an idea for my personal approach to pre-finishing.
Generally speaking, I don’t like finish to get on any mating glue surfaces. Using an adhesive like epoxy could certainly work, but for the sake of nice tight joints and a good wood to wood bond, I prefer to leave the joints with no finish at all, even if that means trickier finishing later. A perfect finish is meaningless if the piece falls apart.
I also don’t recommend finishing first, then cutting the joinery afterward. All of the machine operations are likely to cause scratches and flaws in the finish. Furthermore, the finish itself adds to the thickness of the stock. And even though this is a very small amount, it could throw your measurements off by a hair.
So in my opinion, the wisest way to pre-finish is to carefully mask off the joints. Blue tape works great for this! But in some areas, it can be tricky and/or extremely time consuming. So here’s one pre-finishing method that seems to work well for me. I start by doing a partial dry assembly. For a bookcase, that means assembly the case without the back panel. Then lay the bookcase on some sawhorses and finish the entire inside. Because the back is removed, you will have easy access and the overspray will simply fall to the floor. All dado joints will be masked simply by virtue of the case being assembled. The only taping you should do at this point is the rabbet for the back panel, and that should be a piece of cake. So its not quite as easy as spraying parts when they are flat, but this is a heck of a lot better than spraying with the back panel in place. Also be sure to loosen the clamps and partially knock the joints apart after each coat dries. This will help prevent the case being “welded” together by the finish.
Finish the back panel separately and the entire inside of the case is complete. This is the method that works for me and I am sure it will work well for you too.
Anyone else have some helpful pre-finishing advice for Jeremy?