i need to put a skim coat of bondo on sheet metal surfaces, can bondo be thinned ? even light weight bondo is too thick - a thinner consistancy would produce that creamy smooth look to the finished paint job.

Regular body filler is designed to go on very thin, but it cannot be thinned. However, if you have larger areas requiring a thin coat, use a high-build primer. This is essentially spray-on bondo.
You apply it using a regular auto body type spray gun, and apply it in layers, one after another. Each layer is a bit thicker than a regular layer of spray paint. It dries very quickly, and very soon you can have a 1/16 inch layer or thicker, which can be sanded smooth.

Since its a spray - on, it needs less work than regular bondo to get a proper contour on certain types of panels, and you can apply it to large areas in the fraction of the time required by regular bondo.

It is intended for filling minor imperfections, such as the pockmarks you get after sandblasting a rusty panel.

Usually, it is a beige colour, so a sanding guide coat can be applied on top very easily .

Any decent auto-body supplies retailer can get it for you.

Look here for more info. (http://www.maddyhome.com/canardpages/pages/waynehicks/chapter_25_priming.htm)

Thank you for that very precise very accurate response. To apply these polyester primers requires a nozzle dia of .087 normal guns dia nozzle is .070 for paint/primer. Price for such is $150 for devilblis. now i do have a cheap gun which i can drill out to required diam and apply said product. that seems to be the only way out of this dilema.

I seem to recall my bodyshop friends kept a older, inexpensive gun handy specifically for applying high build primer only.
They did not spray it on with their normal guns.

IMO you could drill out your old gun and experiment. This stuff is intended to be sanded so I dont think any variations in spray pattern due to a drilled nozzle is going to matter much.