With your test spot you will always start with the least aggressive method first, say your polish. If that doesn't work, then try your compound...and if that works, then use your polish again to refine it. That is the normal progression of how to proceed with a test spot. The idea being that you need to find out what products will remove the oxidation and which will restore the surface to like new. Better to do that in a small spot than to try and do the entire boat with a hope and a prayer then be disappointed with the outcome and waste your labor. Work smart!
With gel-coat, since it's harder and thicker than paint, you should be fine to start a bit more aggressive than you would on paint, so continue on speed 6, orange pad, with your compound. Just keep your area around 1ft. square or smaller.
Show us a picture of the entire boat...in other words, not so close. It might help others make a better suggestion by seeing the entire condition of the boat.
If your orange pad and UC doesn't work you're likely going to need to get a product with more cut, perhaps a more aggressive pad, and maybe even a rotary or even all of these at once. From what I'm seeing I think FG400 is a good suggestion if you have that product, but don't rush out to buy it if you don't because I think you'd be better served by products intended for use on boats...of which here are 3 that work the same as FG400 but are likely to be more aggressive.
Products that work like Menzerna FG400...
captains compound, one step boat compound polish, compound for black boat
3M Marine Compound & Finishing Material - 06044
Meguiars M67 One Step Marine Compound
The down side to these is they're expensive with exception to M67 which is on sale right now for $22 dollars a quart here at AG...I think it's on sale. Expense along with the fact that you should only need them once, unless you're planning on doing more boats, is another factor. Whereas IF (big ? of if) FG400 would help you, then you could use it on paint as well. Most recommend against using Marine cutting products on paint because they're too aggressive.
If I had some FG400 I'd send you a sample to try on a test spot, but I don't have any at the moment. Perhaps someone else could send you a sample. I think you'll find you're going to need it or one of the 3 even more aggressive products I mentioned. Maybe not without seeing the entire boat and some photo's that aren't so washed out with white.
Are you considering buying this boat? If so, providing a couple more pictures may save you the trouble of doing a test spot to see if it can be fixed. I'm confident it can be. Or, are you wondering if the owner will let you do a test spot before you buy products? If he won't then I'd pass on the job because that's your tell-all of what you're in for...gives you a good indication of how tough, or not, this job will be so you can figure your labor. Guys with more experience can probably do this without doing a test spot, but I'd never try it...I always do a test spot.
