texture painting with aplha

Id like to be able to paint on my models but there is always a big white blob around what I’ve painted.
I have been able to avoid the big white blob by having a solid background on the png file Im painting on but this way the part I painted effects the whole model or uv map instead of just the part I want to paint.
Im trying to paint dirt onto a chrome part of a car. If I paint the dirt over alpha it gets the white stuff around the part I painted. Using the workaround of having a grey background affects the chromes color and reflectivity.

In the texture settings for your .png try enabling the ‘Premultiply’ tickbox