Hi,
You're most welcome rlouie.
I was awake in the early hours this morning thinking about projects I'm doing or intending to do and your problem popped into my head; it was very quiet and peaceful and I started to think what I would do if the problem was mine.
I'm aware many novices read these threads and I like to encourage novices to leave their comfort zone to try something new. I also try to upgrade or improve on the original whenever possible. I've received lots of training over the last 55 years including an intensive week of Kaizen training this explaining how to take anything right back to basics and start from scratch the idea being in industry not to manufacture a Rolls Royce when a Mini will do.
Applying this to your problem humidity and rust are the basics here; humidity cannot be controlled in this situation so can be worked around; rust can be controlled by choice of more suitable material. Ordinary steel sheet is sure to fail in such high humidity but Stainless steel or Galvanized steel would be better. But as I say taking it back to basics; just because the manufacturer of the unit used a steel enclosure the enclosure doesn't have to be steel or even metal of any kind it simply needs to be impervious to water.
I've learned many skills during my working life one of these being a glass fibre laminator;
Why not take control of the problem and eliminate it completely by making a new fibreglass enclosure; this might sound complicated but even a complete novice can do a decent job. I'm unsure as to the design of your enclosure whether it's a full enclosure or if the bottom is fully open. This could be a very interesting not regarding it as a problem but regarding it as a project and enjoy doing it. A very simple inexpensive mould can be assembled; plywood; MDF; chipboard etc can be cut and screwed; for cosmetics I would design the mould for laying up the glassfibre on the inside so measurements would have to be allowed for the extra size. Internal corners can be rounded by filler and sanded smooth. It would be important to paint every area where the glassfibre would be laid in order to seal the surface.
Double and triple check dimensions too big is better than too small. With the mould assembled and fully sealed with paint then release agent needs to be applied (PVA) and allowed to dry. With glassfibre the finish actually goes on first this is the GEL COAT; this gel coat copies perfectly the mould finish; the gel coat can be coloured with pigment as desired. The gel coat is allowed to "flash off" then the glass fibre mat is applied; For this application I'd use two layers of 2oz mat. The video above explains better than text but a bit of research on the web before starting is advised;
https://www.ecfibreglasssupplies.co.uk/product/600g-2-oz-chopped-strand-matting-1mt-wide
If the enclosure is laid up inside the mould then a nice smooth finish will be produced; if the enclosure is laid up on the outside of the mould then a rougher finish will be produced. For the ultimate smooth finish we had a huge plate glass table where we produced things like shop fronts giving an unblemished mirror finish.
For the louvers measure the total area then lay up as a single sheet and then cut into strips once hardened; for mounting the strips why not add angled ends; the single sheet as a mould would only need a couple of battens one each side then they could be drilled to accept fasteners; imagination works wonders?
We were on production where speed counted; we used to cut a side from a 5 gallon plastic container and fill this with resin using a 12" roller for application; on a hot day it was a case of working just the right side of panic; we weren't supposed to lay up more then 4 oz at a time but we were on bonus and I've seen smoke as the resin hardened.
Great care is needed regarding safety; personal safety means wearing heavy rubber gloves; goggles and old clothes/shoes; we wore wellingtons and boiler suits; at the end of the shift we would stand the boiler suit and wellingtons against a wall; glass fibre laminating is very messy. There is always a high fire risk especially using acetone; we once suffered a fire when two open containers of acetone ignited simply from the suns rays through a roof window.
This needs using with extreme caution;
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Glass fibre laminating is a very useful skill to have and the basics are easy to grasp; mould making too is a most pleasant pastime.
Having said all this I would enjoy making a bespoke glass fibre enclosure but possibly for you rlouie I can suggest an easier option; buy plastic cladding sheeting and cut to size; plain plastic sheet could be cut into strips for the louvers? A frame would be needed and this could be wood.
I derive lots of pleasure from what others regard as a nightmare or problem; novices please don't despair; have a go and come up with some new ideas you won't regret it. If someone tells you it can't be done this is only their opinion; think it through for yourself; there is a huge downside in being able to resolve problems; I'm the one on our street when things go wrong and help is needed.
I've rambled on enough but hopefully provoked a bit of thought.
Kind regards, Colin.