I've done quite a few of these since I'm doing two fastbacks.
I've tried ambient temperatures for up to a week, heat lamps, outside under the sun, in contractor garbage bags, etc.
But the final method I tried really worked well and I was able to do 3 applications of the lye oven cleaner in one day.
For the rear interior quarters (on a 65 2+2) I sourced a 3x2x2 cardboard box. Spray the item and really hose it on. Like a lot of these kinds of products you have to cover the item with enough product so that it forms a skin. I use an acid rated respirator as I cannot tolerate the STRONG odors coming off the oven cleaner.
Put it in the box along with an Edison 100 watt light bulb on the bottom. No LEDs. Cover with the garbage bag or other 3+mil plastic. Do not let the plastic touch the item below. It does not need to be sealed, just covered. Let it sit for at least 3 hours.
Test how well things are going with the brush in one corner. If paint is coming off, take it outside and get a garden hose. Wet it down a bit and use what Amazon calls a drain cleaning brush. If you have two, cut one down so the bristles are around 3/8" long. With the water acting as a lubricant, start scrubbing. The first layer is always the toughest. I use 18" long gloves since lye and skin are not friends. Eye protection too.
Use an air gun, dry and repeat.
I've used the oven lye on 65 and 67 panels so it appears safe across many years. If you intend to do the whole inside of your fastback and some other misc items I would suggest a case of the 24oz size yellow cleaner. The stuff in the supermarkets is usually 14.5 oz.
Here is the 65 panel, all cleaned. This goes into a restomod so ignore the 6x9s. This did take about 6 applications to get everything off. The worst part is cleaning these off outside while it's raining. Plus, it's really messy.
On edit -- Add before pic. The scratches actually ended up in the paint, not the fiberglass.
