Has anyone else had a problem doing this? It seems that the Putty softens.
PPP is water soluble, so yes it will dissolve with water. I will let it dry then knock down some of the access with a damp q-tip, then finish sand with fine sanding tools.
Not being able to wet sand is my only complaint about PPP.
It’s a kind of a big deal to me, I like to be able to wet sand, but I guess we can’t have everything.
Apply PPP, let it set off, then smooth with damp Q-tip, finger, radius tool, etc…
Key advantage is that it can be smoothed while not set, doing most of the work in applicaton, so that you don’t loose rivet details, etc.
Takes a little lateral rethink.
Yeah ;
I just discovered that myself . I had some spots on My Noordyun Norseman and I wet sanded . It all disappeared ! had to do it over .Oh Well , Lesson learned !
Greg, PPP sands smooth as silk. I remove as much as possible with a damp paper towel or Q Tip which reduces all or most of any sanding. Then shoot it with either Future or Aqua Gloss to get rid of any surface texture difference for a perfect paint surface.
Why wet sand?
Thanks for you input, Ernie. You know it is always appreciated.
I think the idea is that the water carry away the sanding debris and loose abrasive material, sort of like cutting fluid in a grinding machine.
Somebody correct me if I’m wrong.
You are welcome. True that Greg on solvent type fillers but PPP is water based as previously mentioned.
Right. I just meant for putties and such that are wet-sandable, Ernie. As we know, PPP turns to mud. 
Thanks for the reply. Just sounds like a mess to me. Wipe down the part with a cloth when done.
Hi;
As an owner and builder of cars in a Auto Body Shop that Repaired rather than replaced parts , Wet sanding was the Cats Meow .
In models it is too .When you wet sand , any large rough particles that would crust up in the grit are instead washed away . Leaving a truly smooth surface. On a real auto this gives a surface like fine satin to the touch , but a heck of a sticky surface for the coming layers of paint !
On a model now . Sanding wet , with 1800 grit or higher gives a silky surface and gives you that Winning surface finish !
Yep, that’s why I like wet sanding too, TB.