Outer Limits 4x4 Board

I am a bit useless with fibreglass so I hope this question isn't completely idiotic.

I started building a custom heater/aircon box a while ago, to fit into a fairly tight space, then put it to one side with other jobs to do. I have made most of it out of light sheet metal and it is fine, but I would like to redo some of it and was thinking of using MDF, but coating the MDF with fibreglass for strength and water resistance. Is it possible to do this?

Basically I am thinking of making some complex shapes out of MDF, then simply wrapping them in fibreglass sheet and painting fibreglass resin over the shapes, so they end up being single-piece items. Essentially I want to emulate a plastic item.

Stupid or doable? Is there any alternative good way of making fiddly shapes?

chimpboy wrote:I am a bit useless with fibreglass so I hope this question isn't completely idiotic.

I started building a custom heater/aircon box a while ago, to fit into a fairly tight space, then put it to one side with other jobs to do. I have made most of it out of light sheet metal and it is fine, but I would like to redo some of it and was thinking of using MDF, but coating the MDF with fibreglass for strength and water resistance. Is it possible to do this?

Basically I am thinking of making some complex shapes out of MDF, then simply wrapping them in fibreglass sheet and painting fibreglass resin over the shapes, so they end up being single-piece items. Essentially I want to emulate a plastic item.

Stupid or doable? Is there any alternative good way of making fiddly shapes?

Perfectly doable - and I've seen it done with sub boxen before.

When you come to do it though, give the mdf a good lather of resin, THEN give it the fibreglass and roll it down, and add more resin if desired. The MDF tends to drink it up. Add some Wax in Styrene to the resin so it fully cures as well.

I'm not 100% on this last bit, but I think Poly resin is better for MDF than epoxy - that's what I've used to stick to MDF in the past anyway, and I think I used it because it holds the MDF better - it stinks though!

Mdf can absorb the peroxide and make it take a little longer to set, or in some cases not set where the resin is thin. I have had great luck with sunbake, cannot reccommend it enough,
http://www.shapers.com.au/categories/Su ... V-Sunbake/" onclick="window.open(this.href);return false;

cheers bru

Using fibre glass over timber (although as bru said, not mdf) is a normal boat building technique that is widely used. If you want to use a timber for ease of purpose then get some ply wood. It is almost as easy to work. Much stronger and just as light. It also works much better with fibre glass resins.

toughnut wrote:Using fibre glass over timber (although as bru said, not mdf) is a normal boat building technique that is widely used. If you want to use a timber for ease of purpose then get some ply wood. It is almost as easy to work. Much stronger and just as light. It also works much better with fibre glass resins.

Good thinking and I have ply already!

toughnut wrote:Using fibre glass over timber (although as bru said, not mdf) is a normal boat building technique that is widely used. If you want to use a timber for ease of purpose then get some ply wood. It is almost as easy to work. Much stronger and just as light. It also works much better with fibre glass resins.

Well there you go!

Learn something everyday.

We do heaps of fibreglass repairs to semi trailer fridge vans were I work. If you want some chopped strand matting for the job sing out and I can send a few metres your way. Couriers / Post might not like the resin in the post though!

tehekho wrote:

toughnut wrote:Using fibre glass over timber (although as bru said, not mdf) is a normal boat building technique that is widely used. If you want to use a timber for ease of purpose then get some ply wood. It is almost as easy to work. Much stronger and just as light. It also works much better with fibre glass resins.

Well there you go!

Learn something everyday.

Basically depending on the type of boat and the design you might build the various panels using glass resin over wood or other light weight materials like foam sandwich etc. then join them with fibre glass. Or you can build the hull like if your making a canoe or something and then cover it with one or more layers of fibre glass to strengthen and waterproof the boat. If you use the correct technique and the right amount of fibre glass then you can still see the timber underneath. Hard to tell it even has fibre glass on there sometimes.

chimpboy wrote:

toughnut wrote:Using fibre glass over timber (although as bru said, not mdf) is a normal boat building technique that is widely used. If you want to use a timber for ease of purpose then get some ply wood. It is almost as easy to work. Much stronger and just as light. It also works much better with fibre glass resins.

Good thinking and I have ply already!

Ply is a far better alternative. If mdf gets wet at all it will swell up, It can swell to many times its original thickness, far from ideal for your intended use

IF you can find Poyurethane foam glue a bunch of that together and shape it to the final shape you want then glass that and dig out the foam. IT shapes easily and it cheap. you can use Styrene foam but you have to coat it with a blocker of some sort to keep the resin from eating it. It is easy to hollow out the styrene though by just adding a bit of acetone and leting it eat the foam away.

zukIzzy wrote:IF you can find Poyurethane foam glue a bunch of that together and shape it to the final shape you want then glass that and dig out the foam. IT shapes easily and it cheap. you can use Styrene foam but you have to coat it with a blocker of some sort to keep the resin from eating it. It is easy to hollow out the styrene though by just adding a bit of acetone and leting it eat the foam away.

Expanding foam works for this, but the only way to destroy it is mechanically! I Actually think expanded foam is polyurethane...

toughnut wrote: Basically depending on the type of boat and the design you might build the various panels using glass resin over wood or other light weight materials like foam sandwich etc. then join them with fibre glass. Or you can build the hull like if your making a canoe or something and then cover it with one or more layers of fibre glass to strengthen and waterproof the boat. If you use the correct technique and the right amount of fibre glass then you can still see the timber underneath. Hard to tell it even has fibre glass on there sometimes.

Oh I knew it was used in boatbuilding, just didn't know that MDF was shyte for it and ply much better

Yeah. Got a little carried away.......

what I have done in the past that looks epic - is cut the side profiles of what you want in ply / mdf, route the edges to a rad, then stretch vinyl over it. I then glass the back with epoxy NOT polyester (vinyl will swell), then you have a completed unit. Much like a speaker pod you can buy for car doors etc,

toughnut wrote:Yeah. Got a little carried away.......

Forums are for learning. I'm sure somebody learnt a thing or two.

In regards to your trick bru, I've done the same, but just stretched some stretchy fabric over the piece, then glassed with cloth (not CSM) and stretched vinyl over that once dry.

Thanks fellas, this is all good info. It doesn't need to look awesome as it will be concealed anyway, it's a fiddly shape just to fit in the space available.

If its an irregular shape, foam would be the go. You can carve it, saw it, grind it, router it, shape it with a power planet to what ever shape you need.
I made fake rocks for a pet lizard enclosure with foam and cement slurry, was easy to shape etc.

80's_delirious wrote:If its an irregular shape, foam would be the go. You can carve it, saw it, grind it, router it, shape it with a power planet to what ever shape you need.
I made fake rocks for a pet lizard enclosure with foam and cement slurry, was easy to shape etc.

So, foam shape, cover with fibreglass, then destructively remove the foam?

Yep, this will give you a rough textured, irregular surface on the inner and outer of your shape,

or if you want it to look pretty, use foam create your shape, (called a plug) fill and sand the surface, paint it, polish it, then cover with fiber glass, pop the foam plug out and use the fiberglass mould you have created to make the finished product with a smooth surface the same shape as your foam plug complete with any irregularities in shape from the original plug

80's_delirious wrote:Yep, this will give you a rough textured, irregular surface on the inner and outer of your shape,

or if you want it to look pretty, use foam create your shape, (called a plug) fill and sand the surface, paint it, polish it, then cover with fiber glass, pop the foam plug out and use the fiberglass mould you have created to make the finished product with a smooth surface the same shape as your foam plug complete with any irregularities in shape from the original plug

I think he's making a one off...but all good advice

tehekho wrote:

80's_delirious wrote:Yep, this will give you a rough textured, irregular surface on the inner and outer of your shape,

or if you want it to look pretty, use foam create your shape, (called a plug) fill and sand the surface, paint it, polish it, then cover with fiber glass, pop the foam plug out and use the fiberglass mould you have created to make the finished product with a smooth surface the same shape as your foam plug complete with any irregularities in shape from the original plug

I think he's making a one off...but all good advice

If you want a smooth, shiny exterior on the product then this is still the better way to do it even for a one off. A lot more work involved though.

Very much a one-off and to be honest, I am a bit flat out so the slightly less pretty but much less work version will be the only option

i used to do conversions on american trucks, we would fabricate a whole new heater, aircon and blend door assembly using the original parts and molded fibreglass parts, they work a treat.
to make the original plug (the one you make the mold from) i layed some 400gsm fibreglass sheet out on a piece of glass (window glass) then painted it through withe resin, it seperates from the glass (window) ok as its such a smooth perfect surface, you can bang a bit of wax on it first to be sure also.
once its set, you end up with a nice thin, strong, workable flat sheet to make the intricate shapes that were needed, you can even bend curves etc. use hot glue to stick it temporary, then a thin bit of sheet and resin over it, or for a few extra bucks, get some plexus glue from a company called fibreglass international in brisbane, its amazing stuff but it does cost a few bob.

we would rivet the end result to the original heater fan and a few other parts, add a drain out the firewall etc.

Stoive wrote:some plexus glue from a company called fibreglass international in brisbane, its amazing stuff but it does cost a few bob.

My cost is about $45 a tube plus $2.50 a 10:1 nozzle for plexus 425. But you're right, our trailers are held together with the stuff

thats a good price, cheaper than we used to buy it? where are you from? pm me if you want

How full on will I need to be with the fibreglass material if I am using ply as the substrate? Will I get away with just painting the resin over the ply here and there?

Is it inside or outside the cab?
How waterproof do you want it to be?
If it fails, how much of a PITA will it be to re-do it?

Are you joins sound in there own right? If your joins are solid you could probably just you just paint it with resin, it might crack along suspect joins.
An alternative to resin could be a polyurethane water proofing compound as used in bathrooms. Bunnings etc would have small kits. there are numerous products, you would do best with one that stays flexible, maybe use some cloth tape to reinforce joins to prevent any cracking.
Lots of these products are water clean up and no stinking fumes too.

80's_delirious wrote:Is it inside or outside the cab?
How waterproof do you want it to be?
If it fails, how much of a PITA will it be to re-do it?

Are you joins sound in there own right? If your joins are solid you could probably just you just paint it with resin, it might crack along suspect joins.
An alternative to resin could be a polyurethane water proofing compound as used in bathrooms. Bunnings etc would have small kits. there are numerous products, you would do best with one that stays flexible, maybe use some cloth tape to reinforce joins to prevent any cracking.
Lots of these products are water clean up and no stinking fumes too.

Thanks, I think you have helped me get my head around the issues... the key will be to have the cloth over the joins, so maybe an approach would be to make the shape in ply, paint it generously with resin, lay some fibreglass sheet over it with a focus on strengthening joint areas that are likely to flex or fail, then paint it over with still more resin.

Does that sound logical? I haven't done a lot of fibreglassing but I have done enough that I think it's within my abilities if the above sounds reasonable.

Yep. Build box, paint with resin, lay fibre glass cloth over it (or just on the joints), soak cloth with resin.
You can add more layers for more strength if you think it needs it, you can do several layers at a time, saturated each layer of cloth as you go.

Thinking about this a bit more, a polyurethane compound would be easier, if you aren't relying on it to add structural strength.

A disadvantage of fiberglass is the cloth or chopped strand Matt won't want to bend neatly around a sharp 90degree corner, it will tend to lift away from the corners as the individual fibres are glass, and won't bend sharply or uniformly on a corner. Soft rounded large radius corners are easier to deal with

I have used Gripset 51 on ply before. Available in 500ml tubs

http://www.gripset.com/product_categori ... ategory=80

It's a rubberised polymer, brush on, dries to a firm rubbery consistency, water clean up, you can use a soft fabric tape to reinforce joins, it will conform to corners easily.
You could make up your ply box, use a hot glue gun to tack it all together, or glue up with Sikaflex, brace it temporarily if needed, paint a membrane on the outside, remove temp braces, coat inside with membrane, all done for maybe $50

R/C model airplane builders do that sort of thing all the time, using fiberglass cloth soaked in epoxy.

Gotta move fast before the epoxy dries, though.