I used a schutz gun. Designed specifically for this purpose- buy them at an automotive paint shop- not your supercheaps and cost about $35.The body deadner comes in 1l tins which screw on to the gun, about 80psi air to get it moving smoothly and away you go. prep is vital- the overspray gets in everywhere!! Take great care with masking off everything you don't want overspray on- I found out the hard way, comes off easily enough but was another time consuming job I didn't need at the time.
Is also available in brushable but needs a litte turps added to thin or very heavy going with the brush. I went for the gun because I did all the underside, inside double panels, quarter panels boot etc. If you only have small areas to do brush might do the job. Gun needs to be cleaned and blown through as soon as job done or you will have to soak to clear internals if the stuff goes off!
Cheers,
Len