undercoat ?

[chubby]

Hello 

I ve begun preparing the cabin sides of my 58ft trad for a DIY repaint . In 6 or 7 places i have some rust . Very light rust , literally at the surface only & not amount thats needed gouging out etc . Ive managed to rub it back with 60 grit paper and then apply fertan . Once washed off and dry ive then added two coats of primer . Where some of the surface is a bit pitted ive filled with isopon p38 . This has also been primered . 

I think , in total the entire surface area of the 6-7 patches must be about 4 or 5 square inches - on one side of a 58ft trad . The other side paintwork looks in good condition so im expecting much the same amount . Overall , the current paintwork is sound .

So - having primered the patches my question re undercoat is do i need to undercoat the entire side or is it ok to use undercoat on just the bits where there is currently just primer ? Where the paint is in good condition  i was planning on a thorough sanding to provide a key for more topcoat but i doubt its wise to topcoat onto primer . I'd prefer not to have to undercoat the entire side due to cost and time so undercoating just the primered bits would be great if considered suitable by those who know .

Im not looking for a super shiny finish , more workmanlike - a tidy up if you like  , but one that will last .

Thanks for any opinions 

 

Edited August 14, 2017 by chubby

[DHutch]

I think it depends on the paint used, the primer and undercoat in question, etc. But if the surface is suitable to paint onto, and the proposed paint suitable to cover what's gone before, there is no need as such.

Certainly I wouldn't slap grey primer over the lot, but often undercoat has a lot more pigment than the topcoat and can be cheaper, so an undercoat of a similar colour is not uncommon.

Daniel

[bizzard]

Also after priming, undercoating and flatted down ready for the top coats,  T cut all around beyond the prepared areas, about 6'' or so that the fresh paint blends in and matches the old paint better without leaving a dirty joining mark.

[chubby]

Thankyou chaps

So , it would be ok to just undercoat the patches then . Trying to " blend " the patches so they don t show is a concern but i will do as you suggest Bizzard. Once done i ' ll give the areas Tcutted a thorough wash down before considering topcoats 

cheers again 

[DHutch]

All depends if the original paint is shot, sound but beyond t-cutting, or just slightly off bright with a few scratches.

Ive not see it, but from what you are saying you are well on the way to getting the damaged areas up to par. If the current paint responds well to cutting back that might be enough, but it might be a case of rub it down with soaping water and fine scotchbite before giving it one coat. Many get good results putting it on with a 4inch roller and laying it off, we use a four inch roller but dont lay it off....

... works for us but it depends the paint being used and what finish you are aiming for.

Daniel

[chubby]

Cheers

The current paintwork is sound . Just a few areas that are not and in those areas - 6 or 7 of them the paint has slightly bubbled . When scraped back it reveals very light rust literally at the surface . With 60 grit paper and a few minutes of elbow grease the patches were basically back to steel . I then fertanned to neutralise any last light rust . Then primered . 95 + % of the cabin is sound & the rust patches are at the bottom corners of the bus type caldwell windows & a few elsewhere & 1 patch around the fairleads ( ?) 

I do indeed intend to put the paint on with a 4inch roller befor laying off . Ive even bought one of those pricy purdy brushes for the job . As i say , im not looking for a showroom finish but i ll still try to make it as good as i can . My main concern was undercoating the primered patches and whether i could get away with just undercoating over the patches as to do the whole side seems unnecessary  ( as the existing paint is sound ) , costly and will take more time . I'd do it if i had to but i wont if i dont . I shall get just one tin of undercoat and apply it where needed and take steps to flat it enough to " blend " with the rest as best i can .

cheers again 

Edited August 14, 2017 by chubby

[WotEver]

15 minutes ago, chubby said:

I shall get just one tin of undercoat and apply it where needed and take steps to flat it enough to " blend " with the rest as best i can

When I used to muck around patching up cars I would blend the new into the old starting with something like 200 wet-n-dry and work my way up to 600 or 800 grit. 

[chubby]

Thankyou again