I've seen a couple of threads with questions about masking and bleed-through when painting, so while I had the chance, I thought I'd do a quick how-to on how I like to mask for multi-color paint jobs.
The first thing is to have the right tools. There's a lot of personal preference here, especially because as tools age, they develop quirks. My tweezers don't have even points, but I've learned how to use it to my advantage. Jerry would probably hate them--it's important to find what works for you.
Note: I'm not going to go into the painting too much here, since everybody has different tools and paints. This is really about masking.
Anyway, here's what I use:
Glass tile (left over from set on Starsky & Hutch)
X-Acto handle w/#11 blade (I'd use Testor's if I could find them out here--sharper than X-Acto, IMHO)
Small Fiskars (mine are NOT Chinese...don't know if the new ones would be worse)
Single-edge razor blade
What tape you use isn't critical. I've met amazing modelers who use 69 cent rolls, but I prefer to get a high-quality 3M Scotch tape. The tack level doesn't matter too much. Today, I used 3M low-tack drafting tape.
Here's the victim...er, model:
Since this is two-color, the first thing to do is prime and get your first color on. I got lazy here and used Rustoleum light gray spray primer from a can. Since I'm painting yellow first, I also need a white base, which was done with a dollar can of flat white spray; Rustoleum white primer is also good, but Valspar is not. But I said this wasn't about painting, so we'll skip to getting the yellow blown on. Getting here from bare plastic took me less than an hour with all 3 coats.
I'll pause here to say that since we're doing multi-color jobs, if you keep working at a steady pace and don't set it down for a week, the first color will be dry but not cured. Be careful not to mar the finish with your tools or fingers by doing something like I'd do (i.e., stupid). Back to your regularly-scheduled how-to.
We use the glass to manipulate and shape the masking tape because clean glass won't remove the adhesive from masking tape. So, start by laying out a piece of tape on the glass plate.
The most important step in masking is to cut a new edge to the tape. Never use the factory edge because it isn't meant to be 1:87 sharp. Use the straight edge & single-edge razor to trim off some small amount. I like to use a tweezer or blade point to pull the scrap away to where I can grab it with my fingers. Knife and tweezer points are extrememly helpful in moving things when you can't get your fingers in to do the job.
Think about where your critical edges are. We know we have to overlap tape edges, but we want to try to avoid overlapping too many layers at the critical color separations. For this job, I started masking around the front intake radiator, partly because of the tight-radius curve and partly so the body side separation could wrap right over the nose masking.
Here's an in-progress shot:
If I have to mask the width of a panel or feature, I always work it with at least two pieces of tape so that I'm only working one critical edge with any given piece of tape. You can see where I have one non-critical seam in the middle. The two sides are made with the blade-cut edge of my tape, and the top edge of the two large pieces does not reach the top of the radiator grill. When you set the tape against the edge, use your tweezers or finger nail to push the tape down solidly against the shell. It's critical for getting a good seal.
The top edge is the second most difficult edge on the whole paint scheme. I used the straightedge and blade to cut myself a narrow strip and started by fitting it against the bottom of the top batten. I let it overhang across the tight-radius curve. This was an unusual separation because I actually had to cut the masking ON the model to get a good fit. Using the #11 blade, I gently sliced the tape to match the curve using a piercing/sawing motion. Slicing will twist and drag the tape out of position. When you've got it cut clean, pull the scrap piece out with the tweezers and poke the edge down with tweezer points to seal it. If a gentle tug with the tweezers shows your cut isn't clean, don't tug/tear. Cut some more to make sure you've got a good knife edge to the tape.
The next step is easy and satisfying. The long separation on the side needs a big piece with a cut edge. Since the Baldwin shell has a physical ridge that sets the color separation, it was easiest to start by laying the middle of the piece against this feature. But when we get to the edge of the ridge, we have to freehand it across to the nose. I'm comfortable enough masking a straight line that I didn't double-check with the scale rule, but why should you follow my bad example? Now, every time the tape goes over a bump or feature, it needs a crease. In order to stay straight, it's important to crease the entire vertical length of the feature or the tape will begin tracking crookedly. Your thumbnail should work fine, but you can use the knife or tweezers too. It is important to be very precise and through, pushing the tape ALL the way into any creases.
Here's what it should look like done correctly:
When the tape wraps around the front end, use the scissors to trim off the excess so that we're not wrapping around onto the other side. When you mask the other side, if you've done it accurately, the two pieces should overlap perfectly on the nose. Mine were within a scale inch of each other.
Meanwhile, another little trick I use is to add all the grabirons before painting, but pull them out extra-far so that the paint gets onto the shell behind them. This way, they get a good airbrush finish with no fuss, and we can set and glue them correctly when we're done painting.
As we get into more rectangular pieces, I'll slice them off the long strip on my glass plate with a razor blade. As I'm holding tape, I'll adjust the shape with scissors. Most often, I'm making sure that corners are at less than a 90 degree angle, so that again, I'm only working one critical edge with a piece of tape. Occasionally on a building or something less critical, I'll try to cut right angles off the tape with scissors, but I don't mess around on a diesel. Here, you can see how I pieced together an inside corner using multiple pieces of tape. The top of the window seal is actually what I'm taping to for the color separation--but since I sealed the edge well, I don't need any more surface to grab and keep a good finish. Remember, each time you overlap a piece of tape, you need to seal the upper one the same way you did going over hood door bumps and other things:
Now, another hard thing about this shell is that the color separation meets at the back corners of the cab. I do this with an overhang of tape since it's too time-consuming for my tastes to try to fit it exactly. BUT, watch for bumps. Here's an example of a difficult lay-down that keeps trying to pull up, and we need to know where these are before painting:
This is exactly where your bleed-throughs will start, and I hate back-masking to clean up color separations. If your masking is done right, you'll almost never have to back-mask. But regardless, pay special attention to the lantern holder bump here, because the tape doesn't really like to stretch over a round feature without pulling up just where you can't afford it.
The other tough separation is around the bottom of the rear headlight. Since it's curved tightly, we know we can't force the tape to curve. We also know that cutting it exactly is a ^@#$%. So, how? Start by cutting a rough u-shaped piece in your tape. It doesn't need to be exact, but should look like this:
This is where you'll really appreciate working on the glass. To do the final fitting, use the tweezers to lay it in position. Now, you can force the smaller piece to curve correctly, and if you need, you can split it with a knife moving away from the light housing so that you can curve it tighter or sharper if needed. I found it easiest here to cut a few rough u-shaped pieces and take the best-fitting parts of each.
Lastly, the rear cab windows need to be masked from the INSIDE of the cab to protect the yellow finish, since we'll be spraying green from all angles onto the shell. Trust me on this one--it's not a wasted step.
Give all the edges another once-over with tweezers, thumbnails, or crowbars, and now we're finally ready to paint:
Don't wait a day if you can avoid it. Paint right away while the masking is fresh and sealed. After painting, give yourself 5 to 10 minutes and get right at removing the masking. Again, best to pull it off while fresh. This is my favorite part of the hobby, practically--watching a multi-color paint scheme emerge. Pull back at a very steep angle to avoid damaging either color:
Also, use the tweezers to lift sections of tape like those on the back of the cab or the front radiator grill.
At this point, we can now see that the masking worked perfectly--no bleed-through, good separations, and we don't have to back-mask and respray:
Those tight-radius curves in the nose look fine:
A little brush touch-up on the left side is good, but we don't see any obvious mistakes.
About that masking inside the cab? Here's proof that it made a difference:
This kept green from bleeding back onto the yellow. At a minimum, we'd have to touch up the window frames with yellow, possibly even repaint the rear of the cab. Ask me how I know this.
The whole process took me about 3 hours. I had about 3 hours' dry time between the yellow and when I started masking, but only because I have a life outside the train room.
And just for fun, here's a shot of the completed loco: