View Full Version : Fiberglassing an existing MDF box (waterproof)

06-29-2006, 12:54 AM
I want to fiberglass an existing MDF enclosure.
I have a bit of experience with fiberglass but wanted to do a double check to see if I am on the right page. I searched the forum and found a post where this was asked however the consensus was that the builder simply uses resin to get the visual effect desired after painting.

The primary reason I want to fiberglass my box is to make it waterproof (I do also want the visual effect ;) as it sits in the engine compartment of my boat which sometimes gets wet, on the bottom.

Since my box is a brand new construction, I have perfectly smooth surfaces to work with and almost all 90 degree angles.
Ill lay down my plan and please let me know if I am missing anything.

1. Prepare and clean the surface.

2. Brush on the resin, lay the mat, brush on the resin. (I may use the method of cutting 4 inch wide strips. soaking them in resin and laying them, adequately rolling the excess and bubbles out)

3. Sand, sand, sand. I donít think I will need to use much if any filler as the original surface was so smooth.

4. Paint and finish. (I think I will actually use gel coat as I have found it easy to sand and shine

The things that I am unsure about are:

1. Would it be good to cut a smaller diameter hole for the speaker and the ports so I can lay the fiberglass strips inside the holes to ensure a smooth finish, later enlarging the holes to the proper size and smoothing the edges?

2. Should I lay more than one layer down or will a single layer serve my purposes?

3. Finally, can anyone see any reason not to use gel coat? The reason I was planning on this is because from what I understand, all of the color is mixed in the coat and will come to full gloss when wet sanded and buffed correctly.

Thanks so much for any feedback.

06-29-2006, 05:04 AM
If you are simply looking at waterproofing you can skip using fibreglass mat alltogether and simply "paint" on resin or gelcoat, the mat has little structural benifit in this suituation and makes finishing more difficult than it needs to be., I routinely use resin to water/weather/idiot proof enclosures.

06-29-2006, 05:12 AM
You can also skip the resin and just paint the MDF the desired color and then throw a few coats of glossy polyurethane over it (sanding between each coat working up to 320 grit) to achieve a completely waterproof, glass-like finish.

06-29-2006, 11:48 AM
To save even more time you can use polyurethane that comes in a can, might not have to sand or sand as much. I did this once for another project that was not audio related and it turned out awsome.

06-29-2006, 11:50 AM
Resin would be much a much stronger waterproofer than just a spray on polyeurathane, but, like stated above, there is no need for fiberglass mat. Mat is for adding strength, which you dont need.

06-29-2006, 11:57 AM
Thanks for the input. Just to clarify as i have never used resin as a stand alone treatment. Aside from the waterproof properties, after sanding and painting (correctly), will I still be able to acheive the look and gloss of a fiberglass finish?

06-29-2006, 12:03 PM