Steve at Hemet Valley told me about this method and then I saw Larry use it too and it has saved my butt a few times recently. If you find an area where you can't use snips it's very easy to cut trailer skin with a razor blade.
I've used this method to split a 10 foot long piece down the middle length wise (sandwich score at edge of two 2x6's) and cut a radius around a curved window frame and for the cut with images below. It produces a very clean cut which is especially important if you're trying to slide the edge into a long Pittsburgh seam for instance...
Use a strong utility knife with new blade, SLOWLY score the line on the first pass and then you can speed up a little on the next few passes. It is easier than I thought it would be to follow the line when scoring. After you have made 4ish passes with the blade gently start bending the waste side of the aluminum back and forth and it will separate at the score.
Maybe not the perfect word to use - but in this instance it means using snips to cut through the waste material all the way up to the line you score. You score the entire line first - but making these cuts allows you to bend and snap smaller incremental sections which can be easier than trying to do the entire length at one time.
Indecision may or may not be my problem - Jimmy Buffett