Zinc Rich Primer- prep work

[Kelpie]

Today I took pity on my Wayfarer's trailer and began to attack it with an angle grinder with one of those flappy sanding wheels. The existing paint, in layers of three different colours, was interspersed with big blotches of mottled rust- fortunately I think this is all merely cosmetic.
I have a tin of 'zinc rich primer' which was bought for another project a few weeks ago and then turned out not to be necessary, so is going spare. Seems like good stuff to use.
Question is- to what extent do I need to take the trailer back to bare metal? I can strip the bulk of the paint off quite easily but getting the last 5% out of little nooks and crannies would take ages. It's also going to lead to removal of some decent steel adjoining these areas.

Does the panel reckon I'd be OK to leave the odd trace of paint still on? Obviously nothing loose or flaking.

I'm now off to cough up black sticky stuff. This is not turning out to be a very nice job!

 

[VicS]

No sense in removing sound paint provided it is not hiding rust or is not compatible with the proposed new coating unless your un-named zinc rich primer must be applied to bare metal.

The important thing is to remove ALL of the rust.

Ensure you use whatever pretreatment is recommended in the datasheet for the primer.

 

[Bilgediver]

Today I took pity on my Wayfarer's trailer and began to attack it with an angle grinder with one of those flappy sanding wheels. The existing paint, in layers of three different colours, was interspersed with big blotches of mottled rust- fortunately I think this is all merely cosmetic.
I have a tin of 'zinc rich primer' which was bought for another project a few weeks ago and then turned out not to be necessary, so is going spare. Seems like good stuff to use.
Question is- to what extent do I need to take the trailer back to bare metal? I can strip the bulk of the paint off quite easily but getting the last 5% out of little nooks and crannies would take ages. It's also going to lead to removal of some decent steel adjoining these areas.

Does the panel reckon I'd be OK to leave the odd trace of paint still on? Obviously nothing loose or flaking.

I'm now off to cough up black sticky stuff. This is not turning out to be a very nice job!

As Vic says there is no need to remove good paint and it will be abraded during the derusting process. The zinc rich primer makes a good tie coat between the old paint and new undercoat/paint that you will be applying.

 

[Kelpie]

Thanks for the reply.
The paint is from Teamac... according to the datasheet it "must be applied in direct contact with the iron or steel to be affective" (sic).

http://www.teamac.co.uk/downloads/pdfs/ZINC RICH DATA TM.pdf

Will the tiny bit of metal beneath a well-adhered bit of paint not be given some protection by the surrounding zinc? Just seems that I can get about 95% of the old paint off easily and the last few traces will take forever to get rid of... by which time the bare metal will probably be starting to rust again....

 

[Bilgediver]

Thanks for the reply.
The paint is from Teamac... according to the datasheet it "must be applied in direct contact with the iron or steel to be affective" (sic).

http://www.teamac.co.uk/downloads/pdfs/ZINC RICH DATA TM.pdf

Will the tiny bit of metal beneath a well-adhered bit of paint not be given some protection by the surrounding zinc? Just seems that I can get about 95% of the old paint off easily and the last few traces will take forever to get rid of... by which time the bare metal will probably be starting to rust again....

Actually you will get protection. For many years I have applied Primocon ON TOP of my wire brushed antifoul on the keels of a Westerly and not removed anything more than the loose antifoul. This is primarily a tie coat. The new antifoul goes on top of the primocon . The keels seem perfectly protected comparing them to the tales of woe heard here however the strange thing is that if I feather the paint layers back down to bare keel one can not see any grey layers The mystery is where does the Primocon go and if it does why doesn t the antifoul on top fall off. ??? My experience is that zinc rich primers do work even if they have a film of paint under them . Anyway the existing paint is already protecting the underlying steel in your case.

 

[JimC]

My experience is that zinc rich primers do work even if they have a film of paint under them .

This has been my experience also when using zinc-rich primers to protect iron keels, though I have found that they do work reasonably well above the waterline. I would be interested to know the reason.