I just use a number 11 blade to scrape the paint off where the cement will go. Because it is a solvent, putting cement where there is paint will oftentimes “melt” the paint and make a big mess. While it sounds like a pain, it only takes a couple of seconds and it is a lot faster than masking it would be.
Although I AVOID PLASTIC kits if at all possible I would add one or two comments which makes the material easier to paint and detail.
Most important, WA***he parts with a mild soap and water solution. This removes any contamination which may be present from the manufacturing process. Paint doesn’t adhere well otherwise.
Next spray with a neutral primer. If you are satisfied with the base color of the plastic you can use fine emory cloth to roughen the surface slightly as an alternative. 90% of the time priming is best.
Do paint the detail parts, including doors and windows, on the sprew if at all possible. All kits require some “dry fitting” so expect to have to do some touchup work if you paint the details first.
If you are using brushes rather than airbrushing (much preferable IMO) I suggest using acrylic tube paints thined with alcohol rather than model paints. Be sure to use high concentration alcohol with only 2-5% residual water. Get it from Home Depot or the like. If you decide to use water as your thining liquid, be sure to use DISTILLED WATER, not the liquid rocks from the tap or bottled water. Thin your colors to the opacity of washes. A little experience will tell you how much to thin the color out. Several thin applications will give more realistic results than a single heavy coat. Less IS ALWAYS better than more. If you have the feeling you haven’t put on quite enough you are already finished. Put the structure aside for a day or two to rest and you will like the results better.
I have had good results with both India and sepia ink washes. I thin 10 drops alcohol to 1 drip ink. A wash made from the same color as the dirt on your layout can also be very effective. Same thining ratios apply to start.
Don’t forget rust streaking, all structures made with metal or metal hardware eventually show rust streaking. Use the finest detail brush you can find for this work.
I prefer to glaze the win