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MG Midget and Sprite Technical - Bonnet Rust Prevention

I might be asking the impossible here, but anyway:

I'm re-painting my Sprite's bonnet. It's a bare metal and filler re-spray. As I suspected, removing the paint revealed a HUGE amount of filler - mainly in a wide band across where the front end had been replaced at some time but also extending to areas that didn't need it. That has all been removed and replaced, but less generously - the layer I removed was 12mm thick in places.

I'm at the top-coating stage and thinking about trying to prevent rust reappearing at the front.

I don't understand where all the water that collects in the front lip comes from, but it definitely gets there. I've Kurusted everywhere that I can reach and sprayed zinc primer into the voids. I'd like to finish off by spraying/injecting some sort of rust preventer.

I have Waxoyl, but I wonder whether the heat generated under the bonnet might cause problems.

Does anyone have a successful solution that they've actually tried?

Colin
C Mee

I can't give a 100% successful solution, but this is my thought. The water comes very largely from condensation forming on the underside of the bonnet. Much of it during cold weather when heat from the engine and warm moist air meets the cold steel of the bonnet, especially at night after a run out with the car during a damp day when water will condense out in considerable quantities.

Water then runs down the forward slope of the bonnet and collects in the roll over lip at the front. That is where you need to concentrate rust prevention, but also drill some decent sized drainage holes to allow the water to drain out and give the area a chance to dry.

I've not tried this, but maybe some insulating material on the underside of the bonnet might help reduce the condensation effect, but it would need to be vapour proof and water repellent as well.
GuyW

Aluminium bonnet?

Seems cheap at £315

http://www.mgbreakers.com/New%20Products.htm


You might need to copy and paste.
Dave O'Neill 2

Definitely drill two or three decent sized holes through the underside of bonnet and if fitted, the stainless steel strip. Initially used Dinitrol and top it up annually with Dynax S50.
Your photo shows the bonnet without the finisher but if one is fitted, that also traps moisture so occasionally I've drilled out the pop rivets and treated the bodywork under it. Next time I'll use riv-nuts to replace the pop rivets.
When washing the car, water collects at the front of the bonnet so it's an idea to open the bonnet and let it drip dry - the water runs from drivers to passenger side because of the tilt caused by the single bonnet bracket so I use a chamois/towel to collect it. Could always not wash the car or as Dave suggests buy an aluminium one - problem solved. no rust.
Jeremy MkIII

For some reason, I can't upload photos of my bonnet and bonda primer.

Anyway.

You aren't asking the impossible and you aren't wasting your time.

I agree. It's mostly condensation. So protect the steel against it, and let it out.

This is my once new steel bmc's finest bonnet, fitted 34/35 years ago.

I too was determined it would never rust again; given the cost, which at the time I thought exorbitant.

It hasn't rusted.

Yes to waxoyl internally at the sharp end. But before that, oodles of my favourite rust preventing paint; Bonda Primer. Followed by flooding the internal at sharp end in primer, then gloss.

Plenty of paint, and then sealant under the polished ally finishing trim to keep the rain out completely, then rivetted on.

3 x 3/8" drain holes through the trim and bonnet. 1 central one close to each end.

Voila, no rust whatsoever.

anamnesis

Thanks for the advice, the comments are really helpful.

I hadn't thought about condensation but I probably won't insulate in case that causes other problems as clearance is limited in places.

I had already been considering drilling drainage holes, so that's the next job before I go any further. I've never tried Dinitrol or Dynax S50, so will investigate. As well as getting it into the voids, I'm intending to put it under the trim strip too.

Well spotted Jeremy. My profile photo is from soon after I bought the Sprite and that trim strip was missing. I understood why when the replacement trim piece arrived as the fit was awful! This was down to the poor quality of the repair panel. After trips to inspect a friend's bonnet, I have spent ages filling and filing and sanding so that, while it's not perfect, I feel the fit is as good as I'm likely to get.

The photo shows how things stand this morning, with the trim strip just lodged in place. After being really pleased with the filling and priming, I'm really disappointed with the top coat, which is probably due to the paint being a couple of years old. I'll be off to the shop for a new tin tomorrow.

PS. Sorry about the inverted image - even when I rotated the original it uploaded upside down.

Colin


C Mee

My photo uploads working now.

I never insulated. Drain holes work a treat.




anamnesis

I have made up my own version of waxoil. I melted some candles with a warm air gun and mixed them into some old oil. When cooled down it formed gell. Unlike grease, which is now all high temp melting, my brew becomes fluid when heated and can be painted or sprayed on and is runny enough to get into seams. I appreciate it will not cure rust but it has kept the inside of my doors and bonnet in perfect condition.

Jan T
J Targosz

@anamnesis

another vote for Bonda Rust Primer here - awesome stuff, you only have to search for Bonda Rust Primer to see all the canal boat people sell it.
Malc Gilliver

This thread was discussed between 28/06/2024 and 05/07/2024

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