Transparent brush overlap 'dotting' effect problem

I have a 512x512 texture that I'm painting on with the option to paint transparency. I need to get rid of the 'dotting' effect that happens. i've successfully done this by storring the coordinates of the last pixels I modified in an array, and then not rewriting over pixels that were changed during the last pass, however this made the program take a significant performance spike.

I also tried writing to a buffer texture that displayed above my main texture. This created the desired effect, however when it came time to apply the buffer texture to the main texture, it only counted the buffer texture, ignoring the colors below it. I thought maybe a shader or a render texture would fix this, but I'm not too familiar with either of those. Any help would be greatly appreciated.

I was wondering if there was a more efficient way to have transparency that does not overlap itself.