Ken,
I would eventually invest in a double action, inexpensive, air brush and a compressor designed for an air brush.
There are thin stripe tapes available in the automotive industry. Will they be fuel proof? Possibly.
With thin outlines, the area of the thin outline is generally sprayed first. Then you apply the thin stripe tape to cover that color. You mask each side of this thin stripe tape for the other colors. The last tape removed is the thin stripe color.
With that arch, you'll have many pieces of tape. I make masks especially for scallops, that is, the thin stripe is in one piece and scalloped.
Your model looks excellent! You may not be happy with all those pieces of tape overlapping at the ends of each arch? However, not so obvious with a healthy clear coat. Masking both sides to paint the stripe would be time consuming but the results may be worth the time? Difficult at 1/16" but possible. The stripe tapes are straight so you have to curve it nicely, you could keep it away a bit from the blue for the extra thin white stripe using the blue edge as a guide.
You have to be careful applying paint dry, it may not bite as well as expected.
I cannot answer questions about Polyspan except why I don't use it. I use only silk for many reasons. I Like Krylon color better than Rust-o-leum. A choice I've made having used both of them.
If you send me a tracing of an arch, or two, if they are all different, I can cut your stripe tape in an arch in vinyl. You won't be playing with straight tape and the application will be easier and much quicker.
Let me know.
I would only use a 2K automotive clear, and yes, available in an aerosol can. Thank goodness.
BTW. You did a great job with those checks.
Are we seeing front row on the horizon? Looks like it.
CB