Originally Posted by Jeffzee
If i get it cleaned to where it looks good, and i keep it covered from the sun when not in use do you think it will stay good, or will it just keep coming back regardless?
If it is plasticiser migration and/or depolymerisation (which is what it sounds like) then a wee chemistry lesson might help you understand the long term problem you face.
Pvc is actually a rigid material (they make sewage waste pipes, and window frames from It). It is made up from very long chains of atoms that pack neatly together which gives it rigidity. In its pure form it would not be suitable for making boat tubes from as it is too brittle when thin and not flexible enough. So during manufacturing they add small molecules (plasticisers) in to disrupt the regularity of the pvc chains which makes it flexible, this works well for a reasonably long time - flexible pvc was ubiquitous in the 70s and 80s but eventually it ages, how long that takes depends on how it is treated. UV, heat and age have two effects. first those plasticiser can start to work their way out from between the pvc chains. Generally heat will speed that up, especially if the temp is different inside/outside the material. That will leave the pvc chains more rigid, and the area where the excess plasticiser migrates to (the surface) gets softer, and eventually sticky. washing the surface with strong solvents can remove the plasticiser but you won't be able to control the process so you get just the excess plasticiser and risk making the material even stiffer (if you try it with a very thin beach toy you'll see you can make flexible material go stiff just by washing it!).
It is NOT possible to put the plasticiser back where it "belongs" or reverse the process, and so it is inevitable that you will eventually get tubes that fail and can only be replaced. It's impossible to predict with any accuracy how long that will take...
The second effect is depolymerisation, this is when those long chains of atoms start to break down into shorter chains (which like the spaghetti in a tin doesn't tend to organise itself as neatly as the long stuff you make bolegnase from). Shorter chains are more flexible but also weaker (hence why they don't make the fabric that way in the first place!). Again this process is irreversible. It might be confined to just the surface layer (as it will most likely by UV driven, and as office888 suggests TiO2 which gives it the white colour makes matters worse (actually it absorbs UV very well, but in doing so it generates free radicals which are bad!). If it was "just" depolymerisation I wouldn't expect the tubes to go brittle, but what you would see is that patches wouldn't stick as the material underneath is "gooey".
Hopefully that gives you some idea of the cause, and helps you see why cleaning it off might only be a temporary fix, and why a boat with those symptoms might be particularly hard to patch now or in the future.
Retubing isn't cheap but if the boat is nearly free and is only a tender for short hops then it might be a risk you are willing to take.