It's not modeling clay 

It's made to be spreadable for only a very short time. 

If your trying to build up an area for shape you need to make it out of something srtonger than Bondo.

Get us some pics of what you're trying to do and maybe we can help.

Sounds like your useing spot putty, I think the red is for laquer. If you want to go over an eighth inch thick you need to go with glass or duraglass. If you use duraglass you need to work it as it sets cuz it's like a rock when it sets.

It's not modeling clay 

It's made to be spreadable for only a very short time. 

If your trying to build up an area for shape you need to make it out of something srtonger than Bondo.

Get us some pics of what you're trying to do and maybe we can help.

I have two project where I need/want something less gooey than bondo and with a longer working time:
1) On my boat I have made a whole bunch of interior layout changes. My motor cover is a 1-off with a bunch of big corner fillets . It went well, just took friggin forever cause I had to do one face at a time and mix little blobs of bondo every 5 minutes. Now I've got the whole rear area to do. It's all cosmetic nothing structural (backsides are glassed).

2) My trailer is covered with some kind of bondo/filler put on by the factory to make it look smooth and sexy. Now I have "personalized" it by chopping out some length. I want to finish up these weld areas with whatever filler/bondo they used to make it all look clean.

Make your own Bondo out of Cab-O-Sil and epoxy resin then you can use different epoxies with different set times

Yah buddy!!!  I'm right there with you.  After using all the various choices that the West System makes, there's nothing you can't do with it.  Certainly not the cheapest way but I'll tell you, it's the schit!