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MG Midget and Sprite Technical - Cellulose paint problems

This isn't of course a specifically Spridget matter as it's about paint. I restored my Frogeye steel bonnet 3 years ago and sprayed it with cellulose. It had a bit of filler on it as the welding was rather extensive! I etch primed it and applied several coats of grey high build, using cellulose stopper and guide coats for all the flatting.

It looked great for 2 years and then went downhill. The grey high build seemed to sink very badly, exposing many scratch marks from the initial sanding. The Colorado Red also faded horribly, darkening to nearer Cherry red! So I decided to respray in hopefully better quality red with clear lacquer on top to give some UV protection.

I flatted down to get rid of a lot of the scratch marks, filling the rest this time with acrylic stopper which I'm told sinks less. I left several days between stopping and final flatting. I sprayed the colour coat 4 days ago, and guess what? All the stopped scratch marks showed up again. But on close inspection I could see that they were not depressions but stood proud of the surface - they had swelled up. They seem to have absorbed solvent from the new colour coat. So what do I do? I will need to flat down before applying the clear coat, but will I go through and expose the stopper underneath these projections? After 4 days they have not diminished much. Also if I don't sand through and it looks OK to overcoat with clear, will I get sinking again after another 2 years?

I've been spraying cellulose since 1965 and never had these problems before. What seems to have happened to paint quality over the years?
L B Rose

The paint you get these days sold as Cellulose is in fact Acrylic and behaves differently. I know of a least 1 local club member who has had real problems with this.
Even resorting to an isolating layer didn't show much improvement. Sorry I can't provide a solution but I too would be interested in information because I am close to the painting stage myself and juggling with a 2 pack or Acrylic decision.

Rob aka MG Moneypit

Generally I like to be in control. I had the rear resprayed in 2k after an accident and the result was not good, as the paint shop didn't prepare properly. The paint blistered 5 years later. 2k is not on for home use, and is a pig to touch up, so one is left with celly/acrylic. From what I've red 1k acrylic is more durable than celly and sinks less. It's what is in spray cans, but all the suppliers I contacted said they only sold celly. I wonder if some of them know the difference!
L B Rose

Yes, I agree. Celly is a name that seems to be bandied around a lot but doesn't generally mean Cellulose these days. Paint shops usually just see our cars as old bangers and just slap the paint on. It's difficult to find a sprayer sympathetic to our cars. There are a few members in the trade in our local MASC branch who I hope can put me onto such a sprayer.

Bernie might be on soon. He knows a lot about spraying as he was in the trade before retiring.

Rob aka MG Moneypit

As I'm sure you know etch primer,cellulose primer, as well as polyester fillers and stoppers are all hygroscopic and will readily absorb moisture in the air.If you block down the body with wet and dry and a bucket of water this makes it worse. So unless you're able to force dry the car, top coating will effectively seal in this moisture - which will be reabsorbed by all the primers and fillers and swell, and lead to either raised surfaces, or mapping - where the surface shrinks down. Most data sheets will advise temperature and humidity conditions, but typically these will be above 15 degrees C and below 60 %, so I think if you're prepping and painting outside these conditions you will have problems.You may also get blistering in summertime too.

thermometeractice is to wet flat with minimal water, section by section, wiping with a damp cloth as I go, and finally hosing off. I did use a heat gun gently to ensure it was dry. But I have always wet flatted and never had these problems before. Also the scratch marks only appeared after spraying top coat, so I'm sure they contain solvent and not water. I sprayed during that warm dry spell on Tuesday, when it was 18
L B Rose

When I was asking for paint information recently there was lots of friendly - and appreciated - advice about the problems. But very little hard advice on practical solutions. Other, that is, than recommendations to use a professional rather than DIY, and to rely on their guarantee of quality. I still don't have a comprehensive paint strategy to follow.

The Celly I got from Auto Paints St Helens looks, smells and behaves exactly like the Celly I used 20 years ago. The paint on my car is still pretty good 5 years on but it rarely sees the great outdoors and lives in a Carcoon during the winter!

I'm hoping to keep it in good condition as long as possible as I don't want to go though the whole paint process again for a long time to come - it's hard and time consuming work.

(I'm still mad with myself for scratching the bonnet last weekend after I let the wind blow it over on my concrete drive)

I used the mig welding site for the prep and painting process, I'm sure there are others as well.

john payne

Guy, the problem with a paint shop guarantee is how long it lasts. My paint blistered within a month, and the shop redid it without quibbling. But it's now 5 years later and blistering again - do I have any redress this far down the line?
L B Rose

I have no idea how long a paint shop would guarantee their work. That is what was suggested to me as I was seeking advice too! I would rather do the work myself though, as like you, l have always done. That was as far as l got, although the migweld DIY site is good on painting too.

To be honest this 'sinking ' is both a celly and 2K problem. There are many possible causes but the most commom ones are that the repaired / sanded areas are not cleaned off both with blown air and panel wipe before etch and priming. Another that when filling the filler contained air bubbles, ie not mixed or applied correctly, which will cause the trapped air to expand outwards after paintwork has been applied, most prevalent on a hot day.
and most common that etch is applied over previous celly coats (not a good idea) then too heavy a primer coat applied after this. This causes puddling, ie the first layers of primer do not dry as quickly as the last applied coat, when they do dry they can split the layers above, this is compounded when the top coat is applied and the thinners re activates the previous layers of paint.

I think the best solution is to remove the filler repairs, only fill onto bare metal, and not paint, and be careful with the appication of the primer and top coats.

All of our restos are performed in 2k and blowing in a repair is not an issue.

"only fill onto bare metal, and not paint"

When you say paint in this context does that include etch primer? I had understood that after media blasting to a bright metal finish it was vital to immediately give all the bare metal a coat of etch primer, or rust would develop within minutes in the micropores of the steel. That is certainly what I have done succesfuly in the past, and only then followed up with any filler repairs and dressing that is likely to be needed before the first coats of high build primer.

Yes certainly etch after media blasting, to seal the steel. But really the etch should be lightly rubbed back as it will retain any surface contaminents from the blasting. We normally dress the area to be filled with a 40 grit flap disc to give a rough surface for the filler to adhere. Filling is also important to keep clean, any air pockets should be blown free of dust when flatting, and before additional layers are applied.

Well it has all become terribly complicated! I never took this much trouble back in the 60s and 70s. How was I so lucky so many times?

This time the bare panel was dried with blown air and and cleaned with panel wipe. I don't think I was guilty of any of JL's list of misdemeanors, sound as they are.
L B Rose

This thread was discussed between 16/04/2016 and 20/04/2016

MG Midget and Sprite Technical index

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