Tamiya basic putty softening plastic?

Today I was filling a hole for a door handle for my '32 ford (which will have a progress soon), and when I was sanding my putty I felt something rolling underneath the sand paper and sure enough the plastic was soft and was almost the consistency of caramel. I have never had this problem using this putty. Did I do something wrong or is this bad putty?

Yes, the putty melt the plastic a bit to "stick" on it. Else, it wouldn't be able to stay there. Normally, the plastic didn't come very soft, but it could happen

Your putty wasn't completely dry for starters.
The putty contains solvents which help it bond with the surface of the plastic by melting it slightly and then re-hardening for a firmer bond with the putty as it dries, if you don't allow sufficient drying time you will experience what you did today.

It's happened to me too when I haven't been patient enough to allow enough drying time.

I gave it four hours of drying time but obviously it wasnt enough. What do you suggest be good drying time for Tamiya Basic Putty?

How thick did you lay the putty? I believe Tamiya Basic putty is air drying and therefore only of use on small jobs. Any major work will require a two part epoxy.

Doesn't matter how much drying time you give it. If layed too thick the one part putty below the surface not be in contact with the air so can not dry. The solvents will contine to eat into the plastic with the results you have sadly experienced :(

Not that thick. I have used this putty many times like I have said. For little jobs like this; little holes, ejector pin marks. This is the first time this has happened to me. I think I can still patch it up with some work though. I just wanted to know why this happened becuase in the future I dont want this to happen again(natrually).

I gave it four hours of drying time but obviously it wasnt enough. What do you suggest be good drying time for Tamiya Basic Putty?
Four to Six hours, it should cure. Depending on how much applied.