just got through with something similar
Peevo,
My partner and I took a job 2 weeks ago stripping a grey (battleship grey to be exact) deck in preparation for staining. We are just starting out and are learning quickly questions that should be asked and tests to perform before committing to taking a job.
This particular one, we took with no testing and the homeowner had just bought so they didn't know the type of paint that had been used. After applying stripper, we realized that there were probably 2 coats of paint and a primer underneath. As we fought with this, we were also talking to others in the neighborhood trying to drum up business and a neighbor told us that quite a few of the residents had used an oil based, marine grade paint on their decks. I don't know how to tell if this is what was on the deck we were working on, but we tried a number of different types of strippers and none worked to our satisfaction (plus the cost was going to be much over what we anticipated by the time we had tried 4 different strippers and applied them several different times). Eventually, the homeowner took pity and said that they'd be just as happy painting the deck. Otherwise, I'd be out sanding that deck right now.
So, you might want to make it clear to the customer that you'll agree to take the job based upon your strippers working, and if they don't then you'll use a sander (and be sure to give them your rate for that service before you start).
We didn't try it, but members here suggested trying HD-80.
We tried testing aircraft stripper which is something I have used in past jobs but it didn't have much effect. Good luck in whatever you decide with this job.
John