Need help with patch

[learning to fly]

I was pulling an old patch off from a boat I just bought. It was a crap job and you could tell that the person knew nothing about what they were doing. When I got the patches off I had a hole about 1/4". Its preety beat up underneath and I started grindingh down to the hypalon but underneath the patched I found what seemed like white rubber calk. at least that is what I thought it was. I started to grind it down but this would take me right down to the threads. My question is this, do I continue to grind right down to the threads cleanly or could this this white substance be hypalon. I have noticed that on the back of some material I have for patching there is a white layer, the other side had the grey material that I took as hypalon. Could this white substance be hypalon and am I better leaving the stuff on and just scuffing it and cleaning it than taking it down to th threads? I am just wondering If I am taking off something that I shouldnt. Whats left on there now seems pretty well bonded and might actually help with the adhesion. I will try to post a pic to help with identifying it
thanks

[Stoo]

Is the white a fabric? My grey hypalon is attached to a white fabric. It is my belief that when you apply a patch, it works best "rubber to rubber"... You really shouldn't grind down to the fabic!

[learning to fly]

no the white feels like a calk, it definitly is not fabric, it almost seems like rubber

[Nos4r2]

If the previous owner was a bit clueless it might be something he's stuck in there to try and seal it?