This isn't a classic car question as such but it's something that classic owners have to deal with and may have experience with
My mother's 12 year old renault is showing some surface rust on the chassis/floorpan. It's not bad but if nothing is done with it it will probably get worse. So I'm going to try treating it. I think have a pretty good idea of what to do:
1) Clean all dirt and grime from the metal to be treated
2) Scrape off all rust and flaking paint with wire brush and sandpaper
3) Apply "rust inhibiting" primer, rub with fine sandpaper
4) Apply top coat(s) and laquer
5) Apply underseal/waxoyl
Have I left anything vital out. Also, here's a couple of questions:
-is it even a good idea to treat surface rust on a galvanised modern car. Will scraping back to clean metal and repainting actually damage the galvanising which was done at the factory and do more harm than good
-I'd guess that the primer application is one of the most important aspects of the operation. Halfords sell Hammerite primer which is advertised as being a rust inhibitor. Is this stuff any good? I have heard that it's not great and that I should look for something called Corroless Red which is used as rust inhibiting primer on oil rigs Sounds like overkill. Has anyone any other recommendations?
-is this rust treating project of mine something that can be done outside or will the humidity in the air get trapped in the paint and feck things up. Also, I don't have access to any special equipment like sprayers etc., all painting will be done using brushes and spray cans. Will this cause a problem. Obviously, since the areas i'll be treating are under the vehicle, the job doesn't have to be perfect as no-one will see it. However if it goes well the next thing i'll try is treating rust on the upper body that can be seen.
Has anyone any thoughts
Looks like a reasonable set of steps
From what I can gather elsewhere, Hammerite is not good for this application. Try something like POR15 or Chassis Black instead.
Regarding galvanised metal, if the metal is rusting the galv has broken down and is not protecting the substrate. Your treatment would help galv and non-galv steel. You should use an etch primer to chemicaly etch the galv prior to painting with something like POR15. Are you going to do the whole area?
When it is all painted, you might try covering with something like waxoyl, dinitrol or fluid film.
Thanks DS. Do you (or anyone else) know of any good shops for buying paint/primer/rust treatments such as POR15 etc.
I have done some research on the etch primers you mentioned, there seems to be a bit of debate as to whether they're really necessary and whether the hassle involved in applying them (they are apparently very hazardous and corossive) is worth it. There is actually a whole loty of debate on the whole area of rust treatments, I have read good and bad reports on products like POR15, Hammerite Anti-rust primer, Jenolite rust killer etc.
Does anyone here have personal experience of any of these products?
Re POR15: You can buy it on the Web, AFAIK. Halfrauds might have some too. I don't have personal experience but a large number of people (mostly American) in the world of old Citroën swear by it. All caution that you MUST stick by the letter of the instructions - particularly in terms of cleaning, etc. You might need to use their other products to clean/prime/etch, etc. and it is not cheap.
I'm surprised that you heard etch priming was corrosive - it reacts with the surface micron or two of Zinc in galvanised to create a key for application of regular primer/paint. NB: It does not remove galv, per se - Galv is normally applied to a depth of >50 microns. I only suggest it's use based on my experience with Galvanised steel cladding products. It wouldn't be wise to use it on non-galvanised surfaces, I suppose.
Another alternative for pre-treatment of the affected area is metal flame spraying (or flame metal spraying). This process involves vapourising a sacrificial anode (like aluminium or zinc) in a gas flame and shooting it at high speed onto the steel substrate. Because it is being sprayed, the temperatures are much lower than galv which means less deformation of the shape and the bond to the substrate is much better. I don't know anyone doing this here, but it is reported to be tougher, lighter and more resistant to corrosion than hot dip galv or electro plating. If you do hear of anyone, will you let me know?
galvanise only needs to be etch primed if it's new, old galvanise has oxidised and will take paint just fine, it's the new shiny stuff you should watch out for.
I'll give a vote for galvafroid, it's a zinc rich primer which gives good protection even when scraped through, kinda like galvanising, but not as good as it's not been melted on
I don't know about painting onto oxidised galv. It'll take paint grand, but I don't imagine it will hold it......
As regards POR-15, according to their website it's better to apply it over a slightly rusty (but not flaking) surface than a clean, shiny surface. It does sound quite good. Halfrauds don't seem to stock it though.
I still don't know about the etch primers, will have to do a bit more reading
Just found some more info on POR-15
Tips on use
Nice "starter pack" including degreasing and etching solutions
UK based distributor - doesn't seem to offer the starter packs and prices seem a little dear.
Stay clear of Hammerite...not good for the adjoining paintwork.
When galvanise oxidises it forms a hard surface layer which protects the underlying zinc from the elements, this layer is rougher than fresh galvanise and will take paint.
the alternative is to etch prime, but that's not always a success if it's not done right and yes, the primer is nasty stuff, it is basically acidic paint that corrodes a rough surface onto the zinc and sticks to that