Cutting dadoes and spraying
Finding the best order to assemble, finish and dado. (Cabinetmaking Forum) May 1, 2003
Do you finish cabinet sides and other parts before dadoeing or after? Currently I cut the parts and dado, then finish, but I know that the finish can damage the effectiveness of the wood glue. On the other hand, dadoing after finishing seems to always produce scratches.
You might try Roo glue if you finish after dadoing. It seems to hold better on finished-surfaces-to-wood joints than our typical wood-to-wood glue does in the same application.
We use pre-finished ply for some jobs, dado with a panel router, and don't seem to have scratches to worry about. If there are any, they are few and small. Besides, we are talking about cabinet interiors, not the exterior that shows much more. I prefer wood-to-wood joints with aliphatic resin glue because I know how strong it will be and don't need to worry.
I've taped off dados when I don't want finish in them. Use a bit wider masking tape than the dado, sticky side up and create slightly convex cover from within the dado.
Based on your question, am I correct that you are finishing before assembly? If so, why don't you assemble first, then finish?
From contributor G:
We mask off 3/4" wide dodoes with 3/4" masking tape. Takes 10-15 minutes to do an entire kitchen.
From the original questioner:
Never thought about masking! Yes, I cut the cabinet sides and all parts and finish before assembly. I assumed I would get too much overspray trying to spray inside an assembled box. Do you spray after assembly?
From contributor J:
I have always assembled then sprayed. I leave the backs off when spraying - can get into almost any cabinet that way, regardless of depth, width, size, angle, etc. I am a smaller shop, though, and last thing I need is another 100 pieces floating around during the finishing process; doors, drawers, and drawer fronts are enough as it is.
I don't understand how you guys are using the masking tape. Are you only using the 3/4" on the bottom of the dado? I'm guessing some finish is getting on the sides of dado, rendering the valuable surface area ineffective for the glue bond?
From contributor G:
I only mask the bottom. My dadoes are only 1/8" deep. Even with deeper dadoes, glue on the bottom only offers more than enough strength. Some shops don't even use glue. Some shops don't even dado. We also use pocket screws with the dadoes.
My dados are a little deeper than contributor G's, so 1" tape works for the bottom and up the sides, and I pre-finish one step. Depending on the topcoat, I spray the vinyl or NC sealcoat or shellac to the full panels before any cuts or joinery. That way I've got some protection against minor damage during the rest of the process.
I find what contributor J does works for me as well. Bracing the cab backs with scrap ply 45 degree blocks, 10-12" as needed to hold everything square and spraying the interior finish coats is a whole bunch easier than doing it with the backing already in place!
From contributor D:
Why not cut a few strips off a scrap plywood panel to set in the dados and grooves? After spraying one panel, remove them and put them into the grooves and dados of the next panel. Should only need a few different lengths; basically a set for base cabs and one for wall cabs and maybe a different one for odd sized cabs.
From contributor G:
I tried that but the strips get crappy fast or blown off while spraying or bow. I went back to tape.
I have always assembled first then finished the assembled units without the backs on. I find it requires less part handling time. And what about pushing the finished parts through the table saw. Don't they get scratched, even a little bit?
From contributor D:
Here's another idea. Spray your panels all the way up until just before the final coat. Machine the joints, then scuff sand, which will remove any minor scratches, mask the bottoms of the dados/rabbits, and then apply the final coat. If you keep your gun perpendicular to the surface, with just one pass of the gun, very little (if any) coating will get on the sides of the dados, rabbits, and grooves and you’ll have no problem with glue holding the joints.
I sometimes use pre-finished plywood. When a panel gets scratched (badly enough) during machining, I will scuff sand it and give it one clear coat to dress it up.
When I do use dados, I assemble, leave the backs off, and spray the inside of the boxes and backs separately, as some have suggested. The one drawback to this process is that it takes a little extra care, as the cases are more prone to racking and joint loosening until the backs get applied. However, if you spray from top to bottom, overspray is virtually eliminated.
When finishing after dadoing but before assembly, why worry about getting finish on the shoulders of the dado? The surfaces of the panel you are fitting into that dado have been finished so glue isn't going to hold to it anyway. Or am I still missing something?
For heavens sake, assemble the boxes first! I only do the “put the back on later” route when an open bookcase is being finished. Otherwise we always put the backs on first.
Get yourself a Kremlin airmix system that will get a bunch of material on the work in little time. On larger units, a dash of retarder goes a long way.
I've opted for finishing everything flat if possible because it's a lot easier for less experienced finishers to get up to speed than spraying vertical surfaces. We just use Roo glue for everything pre-finished and have had no trouble. If you ever doubt the bond of Roo glue, glue a piece of plywood flat to the face of melamine with it. When you pull them apart it'll be the bond between the plys that gives long before the Roo lets go of the melamine.