Considering you admit to NOT being a woodworker let me suggest this method for accomplishing your goals yourself (it may require tool purchasing/upgrade but will lead to repeatability in the future, you did say you wanted 'hundreds of each size'):
First - Priming the panels:
- Prime the sheets as your very FIRST step. Thin coats, NOT heavy. Reapply per instructions, get to an even thickness that is thick white in color. Trust me, prime first, cut second. It's faster and produces cleaner results
- Use a roller insert that is designed for the viscosity, I used to use 4" foam roller inserts for this as they left little or no texture. You don't want an insert that will leave texture (rough surface), unless that's your goal.
- Priming the back side at least once is a nice thing to do, it helps prevent warpage that's caused from the applied finishes shrinking.
- Use an oil based primer, NOTHING water based. Possibly start with a sanding sealer or thinned lacquer (doing so will help you better prevent staining and brown color bleed through that's caused by the wood content in the Masonite).
- If you desire a DEAD flat panel, prime (or spray paint) the last layer with a light colored/light gray tinted product (see sanding below). There are products designed specifically for this purpose (check automotive finishing products).
Second - Sanding the panels:
- BEFORE beginning to sand, make sure the primer is thoroughly dry. If it feels tacky, I'd wait until it isn't (especially if you're drying them in the humidity - in the garage, etc.).
- Sand the primer coat with a random orbital sander, preferably a 1/4 sheet type (which will save you money on sand paper). Make sure the sander is equipped with the OEM pad, that it's in good shape (flat) and that it's a firm pad. You don't want to use a pad that is dented or chewed up as that produces uneven results. You don't want a pad that's too soft or thick as you'll get an uneven surface.
- Preferably, sand outside or with a vacuum attached to the sander.
- When sanding a finish I prefer one designed for just that, usually referred to as a 'non-loading' and/or 'waxed' sandpaper. You can use the regular tan 'woodworking' papers but I'd upgrade to a 'finish sanding' paper as you'll save in the end. Most manufacturers make a range of paper types and most have finish processing papers in their offerings. The best 'non-loading' sandpapers, designed for use with finishes, will contain waxes to prevent the finish (in your case, the primer) from clogging the paper (AKA 'stearates').
- sand lightly, let the paper do the work.
- When sanding DO NOT press hard as it doesn't help, it is counterproductive. Let the sandpaper do the work. Pressing down will generate more heat which will increase the likelihood of loading up/clogging your paper.
- Before sanding make sure you know how to sand. The method best to use is something like mowing a lawn, with overlapping lines. It's best to sand to the panel edges. You'll sand Right to Left on the first pass and then perpendicular to that on the second pass - Top to Bottom. Repeat as necessary
- As you sand, run your hands across the surface, it helps feel for flatness. Looking at the finish at a low angle, into a bright light source, is also a good way to detect surface texture.
- IF you want texture in your finish, sand appropriate to the finish level desired.
- If you chose to go for DEAD flat and primed/painted with a last layer that's gray - once the gray spots are off, it'll usually be dead flat.
- IF you have a hard time sanding large panels, cut them down to a manageable size first. I recommend sanding the whole panel prior to cutting for best results. If you cut and then sand you create a risk of 'sand through' or 'sander round over' at the edges. Sanding prior to cutting produces the cleanest results.
Third - Cutting the panels
- Cut the panels into strips on a table saw. Use a saw blade with plenty of teeth, the more teeth the cleaner the cut (40 teeth minimum, preferably 60+). And make sure your equipment is properly squared up - that's the blade, the fence, etc. - all of it. It's not only safer that way, it also cuts much more cleanly.
- Cut down the strips, the ones you made with the TS, into smaller sizes using a miter saw. You will save time by stacking like-sized strips neatly on top of each other (make sure they're plenty dry = not tacky). Doing so will mean that you are cutting more than one at a time.
- When cutting down with the MS, use a 'stop block' to replicate your desired size. Using sacrificial material on the bottom will help keep the roughness at the bottom to a minimum.
- Also use a carbide tipped blade, in your MS just like you did with the TS. You probably can use the same blade that you used on the TS, if they're the same size (typically most home use products are 10").
- If you don't have a miter saw, you need one. You can pick up a miter saw at the flea market or Craigslist for under $50. Your neighbor might have one if you don't.
Fourth - Final Finishing steps
- I don't recommend the router for the edge, your sawn edge should be adequate (better blade, better cut).
- IF you have to sand the edges I recommend stick down sand paper on a very straight surface or block, one that is preferably longish. This is tough to do without un-squaring the sides of the panel in relationship to the face. You can avoid having to sand the sides of the panel by simply painting them as you put the panel to use in your painting.
- To remove roughness at the edges of the face or back, I recommend you 'break' the edge with sandpaper. You can do so with sandpaper held to a solid block (hardwood, etc.), held at 45 degrees to the surface (look up 'breaking an edge'). Doing this will produce a very light chamfer which will clean up the face and back nicely. You want to use even pressure and produce a consistent result for evenness of appearance. There are handheld chamfer planes and also burr removal tools that can be used for this purpose.
Sandpaper Recommendations:
- 3M makes the fabulous 'Sandblaster' series, any of the orange-ish colored, middle grit # (180 - 320 grit) are designed to sand finishes, I used that type when I repaired tubs (sanding sticky gelcoat). The paper can be used on a 1/4 sheet sander or with a block. The Sandblaster papers are available at most DIY centers and hardware stores.
- Klingspor makes the excellent PS33W papers, that's even more awesome for sanding finishes, though their PS33 does a more than adequate job. The Klingspor Woodworking Shop in Raleigh sells their papers at a discount to NCWW members
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I hope I haven't tread too hard on your present level of knowledge, forgive me if there's too much you already know. In the past I was shop manager for a sign company we did a lot of signfoam panels. Those required a DEAD flat primer coat, especially when finishing with an automotive quality topcoat.
Dean
