How do I prepare a VERY rust stained painted boundary wall for repainting

[gdhm]

Hi,

I am in need of sound advice so as to not waste paint, money or my/others time

Our front boundary wall has needed painting for few years (not painted for 7 years). I have put it off because I have no idea how to prepare it as it has widespread rust on it. The rust has come from top-of-wall metal decorative spikes which have over time leaked rust on a large scale.

I have decided to remove them. I never actually wanted them anyway, preferring low maintenance broken glass cemented on top of the 3- 3½ metre wall, as too high to really benefit from the decorative effect. I bowed however to my wife's s wishes as did I with the 3 grills in the front wall. I had planned to have a solid wall as unlike many Thais in my area I like my privacy and prefer not allowing people to watch us from the road. As our garden has matured we can barely see outside to the road through the grills anyway nowadays.

Somehow I think I will find it hard to get my wife to agree to removal of the grills BUT I have insisted the wall top spikes go.

OK here are 3 pictures of the extent on the problem. The cleaner (light lemon coloured area is the result of my scrubbing with a nylon brush and rinsing with a watering can. It took about 10 minutes. Obviously using a hose would have been quicker (but this was an experiment to see if it could be removed ). The lemon area if rubbed with a damp tissue will still release a a little light yellow rust, so clearly 100% of the rust cannot be removed by dish washing liquid water and scrubbing. The darker reddish areas I had previously thought were where the old paint can come off but in fact they are just more badly rust stained areas. The SuperCote used seems to have lasted pretty well considering it is over 7 years old and constantly exposed to sun and all weather conditions (especially on the lower part of the wall where the rust has hardly reached.)

My concern is how do I seal 100% the wall so I can paint Dulux White Weathershield Ultima Acrylic Emulsion over it without fear the rust yellowing will come through either quickly or in time. I have read that concrete sealer primer should ONLY be used on powdery or flaky previously painted surfaces or it may cause more issues than help. The scrubbed surface on my wall although not free of all rust is sound and not powdery or flaky.

I really need knowledgeable advice on what to do that will prove 100% effective in sealing /removing the rust so the new paint is not damaged by rust seeping though in time. Please bear in mind if cleaners/sealers are recommended they must be readily available in Thailand (as outside Bangkok (I hive in Khon Kaen (Issaan)

Many thanks for any solutions offered

Edited October 12, 2014 by gdhm

[ClutchClark]

As you have already determined, first removing the source of the rust is best, and then washing as much of the surface rust marks away with a chemical cleaner like CLR and a scrub brush would be step 2.

Finally, there are exterior primers that are specifically designed to block bleed-through. I use the following:

http://www.rustoleum.com/en/product-catalog/consumer-brands/zinsser/primer-sealers/b-i-n-advanced-synthetic-shellac-primer-white

However, I have used a brand called KILZ with lesser success.

Cheers

[rhythmworx]

Citric Acid powder diluted, you'll probably find it Rajah brand. spray on with a garden type spray can.

[rawhod]

An oxalic acid solution would successfully remove the iron staining...

However, oxalic acid is toxic and may be difficult to obtain.

Oxalic acid is an organic compound with the formula H₂C₂O₄. It is a colorless crystalline solid that dissolves in water to give colorless solutions. It is classified as a dicarboxylic acid. In terms of acid strength, it is much stronger than acetic acid. Oxalic acid is a reducing agent ^[3] and its conjugate base, known as oxalate (C₂O₄²⁻), is a chelating agent for metal cations. Typically, oxalic acid occurs as thedihydrate with the formula H₂C₂O₄·2H₂O. Oral consumption of oxalic acid in excess or prolonged skin contact can be dangerous.

Edited October 13, 2014 by rawhod

[kannot]

ANY "oil" based paint will block the rust bleeding through then paint over with whatever you like, you only need to paint the affected areas, small roller will do, would do it a couple of times.

[Cylon]

I have removed rust stains from old sails using Oxalic acid .

[gdhm]

Hi ,

Thanks for the replies,

Below is some comments, NEW observation of scrubbed area, and a few queries please.

1) Does anybody know whether http://www.rustoleum...ac-primer-white or CLR can be bought in Thailand. We have a HomePro, Global and HomeHub but if none of these sell them or any of the other items mentioned then nobody else will in my area. I do no buy items, except software (occasionally), on-line as I feel if issues arise it is too difficult to deal with in Thailand.

2) I would not be willing to use dangerous or known toxic compounds (even if I cna get them) as they could easily get into the soil and maybe affect fruit, vegetation or trees, or worse harm the dogs, cats and kids that/who walk down our soi.

3) Just before I posted yesterday afternoon, I had washed the "cleaned" area with an approximate 30% vinegar/70%water. Being concerned the acid may effect the previous paint or wall render it was only on the wall for a few minutes before I watering can washed it off The pictures in my OP were taken 10 minutes AFTERWARDS. Today I went to check my post box 11:30AM and was astonished to find the scrubbed the area MUCH whiter and virtually the same colour as the unaffected part of wall below the scrubbed test area (see photos which accurately reflect the colour differences between the original photos and the new (on my VDU anyway ) . To be honest I am wondering if the hours of direct hot sun on the wall (we are South facing) is the cause of the bleaching, rather than the diluted vinegar. It currently looks white I am NOW thinking maybe it is close enough (or same) not to bleed through.

Any opinions on this please?.

4) I once used gloss pain to seal a rusty iron rod that was leaking rust to the outside of a white acrylic emulsion painted column. It worked but I did need to try twice and the second time the cot was quite thick (I suppose it was possible the rust was leaking to the side of the glossed area. HOWEVER, I thought Acrylic emulsion cannot/should not be painted on a oil based painted surface because it does not adhere very well. Am I wrong?. (I would need to paint whole wall as MOST of it have rust staining)

Many thanks