You could probably get away with using polly filler or any domestic filler for that matter, all they do is create a solid surface once dry. The main issue I think would be how easy it is to sand down and whether or not it would react with the plastic. I use model filler by Humbrol and it works very nicely and is very easy to sand down, it also dries within 30 minutes. I've never used car filler before but i imagine its better designed for metal as opposed to plastic so the bonding agents would be different.
no do not worry I have found filler. It is called use the spruce left over from the model you are making and file it down till you get the filling from the spruce and use modeling glue old tin for mixing in about two drops and the filling particals filed from the spruce mix together apply till paste like apply to spot where you want it wait till dry then file down, then wet and dry to smooth finish hey presto it looks ok to me but to you might, think it might be c**p your decision. take a look. Here are the tools I did the filler with even though it's a lumpy pastelast pic
i use car filler. it is way stronger to stick into gap and against sanding than scale model fillers. if the gap is big tamiya fillers maight get cracked, wallejo fillers doesnt like sanding etc... also you have another option but it is stinky you can put cellulosic thinner in a tamiya paint bottle and plastic sprues in it. when sprues melted, you will have liquid plastic. it stinks like hell but very strong when it get dried