Welcome aboard to Armorama

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Well Paul here's what I do to get nice straight lines on my canopy's. I'll lay out the lines with painters masking tape, it's a low-tac tape. Cutting the tape in strips helps you comform and follow the canopys frame. Once done I'll go over the seem using a clear coat, most times I'll use Furture floor wax. What this does is that it'll stop the paint from bleeding under the tape. Just brush on a light coat. Once it dries paint on the color of choice, let it dry about 4-5 mins. then pull the the tape.
Other methods are to use Bare-metal foils, found at most hobby shops. Just cut and mask as you would do with tape. You may even find canopy masks that some aftermarket companys offer.