Gelcoat completely bonded and ruined Mold... no idea why

Hi All,

I am perplexed at a very expensive and time consuming mishap that has trashed a 12 foot mold.

The mold came out great, however after shooting gelcoat and skinning the first hull I realized the gel had pretty much bonded to the mold. No amount of scraping, tearing ect can save the mold.

I had followed the same procedures, temps, mekp ratios, waxing ect... from previous mold building that turned out great. All of these materials I have used before and am familiar with.

My only thought as to what happened is that the colored gelcoat (a brand I have used on seasoned molds) was used on this brand new mold. Maybe there was something about it that caused it to bond more than other brands.

I know there is great discussion on just wax vs wax and PVA however I had made several molds perfectly with just wax and thought I had the process dialed in. Frankly with this failure it seams as if I didn't event wax the mold... thats how much the gelcoat bonded.

Does anybody have any clue if a mold failure like this is just bad luck?

 

Usual causes are undercured tooling or wax application error (low ambient temperature or rushed).

Undercured tooling ; low initiator reactivity or low initiator %.

 

Thanks for the info! I think I can rule out the tooling gel. Using the same brand and percentages I have never had an issue with it.

Interesting about the wax application temperature. That is one thing I did not think much about. I did keep the shop cool at 65 while I waxed up to limit the sweating! I have not seen much info on the temp when applying wax... is there some info you can share about that? I spread the waxing process out over the course of two days.

What I am starting to think is there is some differences possibly in the styrene content between the two gelcoat brands. I will check data sheets soon. Today I was able to scrape free a section of the mold and applied the brand of gelcoat I am most familiar with and it peeled right off just like it should.

Lesson learned I should have done a small test batch on the mold prior to shooting the whole thing. And also keep the temps up when waxing.

 

Initiators has a recommended shelf life of 3 months. Minimum % recommended is usually 1.75 or 1.5%.

Wax application;
Sufficient time must be given for the solvents to evaporate and the wax to harden slightly.
Premature buffing is easier, but is likely to remove ALL of the wax, as opposed to some of it.

Apply wax.
Wait 60 seconds.
Run your finger across it.

Repeat this every 30-60 seconds.

You're looking for the point where it wipes off as a "dusty" residue, as opposed to "greasy".

Once you hit that "dusty" point, thats your minimum dwell time at that temperature & humidity.

 

Great info... I had been following that and am pretty good with the patience aspect of waxing.

On the other hand I had no idea MEKP had such a short self life. I easily could have used some MEKP that was more than 8 months old when I mixed up that gelcoat.

I tested both gelcoat brands on the mold in question today and both came right off the surface with ease! Which leads me to believe either the tooling gelcoat was not fully cured and "inert" and I should have left it to cure I few more days at higher temps.... or I used some expired MEKP when I sprayed the gelcoat into the mold.

Either way a tough lesson to learn but theses are two variables that can easily be eliminated.

 

I'm not a mold expert. Let's just push that out there now.

But.

Because I'm not an expert, I always used wax AND release films.

Is this not done as a normal course?

I would think the wax can be poorer, but a double release method would be safest? Not to mention if the wax buffs poorly; the spray film can help level it out. But what do I know about production molds? Zero.

 

Its increasingly common to use a liquid mould sealer before the wax goes on.Each wax has it's particular application requirements and they are often printed on the bottom of the can.Don't ignore them!

 

Another thing I recall, as an amateur, is that the initial mould surface (for me it was a 34'6" x 49" decent sized vac table), needed 5 coats of wax for the initial time. I doubt two coats of wax was sufficient or recommended by the wax maker.

I really appreciate this thread, though, because I had some confusion about when to buff.

I only had one part stick to my table. I used a lot of wedges and a lot of air and it took about 4 hours to remove an 8'x2' panel. Honestly, things had been going so well, I think I got lazy and forgot to wax. Fear became a good friend after that.

 

Mold sealer.... thats interesting products like the one linked here? Kantstik Mold Sealer 1.0 | 1 Quart https://www.sherfab.com/kantstik_mold_cleaner_1qt
I may have to use that also as well. After having ruined such a large mold PVA becomes more and more appealing as well. But having made large molds with just wax we all develop our own process!

 

As Wet Feet says I always follow the wax tin instructions. I was using a product called TR mold release which is very similar to most waxes. On a brand new plug or mold wax it up, let it haze (5-10 minutes) buff off... it becomes like a fine dust. Wait at least an hour and repeat 7-8 times. You can see and feel the surface become a little more shiny and slippery feeling. I had great success with this.

But in hindsight..... I either rushed the first part and should have let the mold cure up a few more days, or I very likely could have used very old MEKP on either the tooling gel or the gel for the first part! Needless to say some PVA could have saved the entire thing had I used it!

I really think running some small test patches on the surface is a good idea... that could have saved me as well.

 

Sorry, I misread that you only waxed two times somehow. (Need coffee!)

I do have one question..

Doesn't mekp solve wax? So, am I not wrong that you are applying solvent to the waxed surface? This was why I always used pva..I was worried about the wax getting solved [sic?].

 

New molds can be not fully cured for a while depending on ambient conditions and other variables.

Plus, if the gel coat you sprayed on the mold doesn't cure quickly the styrene will attack the wax and start bonding to the mold. So new mold, plus slow cure, can result in sticking.

PVA is good insurance on a new mold, or apply gel coat and blow it off. Many tooling shops do a blow coat on everything first, worst case if a few places stick you only need to sand off the gel coat that stuck, no fractures in the mold from wedges and pounding.

 

That would be the sort of thing.It may be best used after a proprietary cleaner that removes all compounding residue.One thing to keep in mind with any moulding situation is that answers will be dependent on the experiences of the person being asked.Or sometimes on the range of products that they sell.You can find that in such cases you will be recommended to use a selection from the product range they represent.Which is likely to yield a satisfactory outcome but there are occasions when products from a range of suppliers might result in a better outcome.The only reliable way to find out is to constantly do your own research and to record the results.Each time a salesman tells you that his new product is the best thing since sliced bread,see if you can get your hands on a small sample and find out for yourself-order some if it really is good and consider it as progress made.

I have been a witness to three almighty stick upsne was caused by using a polishing rag that had been used with one type of wax with a different wax.One was the result of a newly hired laminator,who claimed some experience, and who had been told to wash a PVA coated mould to clean it before making a part.He washed the mould with acetone...... The last and most serious was a deck mould for a 40 foot boat where somebody tried to save time and skimp on wax application;fragments of the plug came away very slowly and laboriously and it took weeks to make good the damage.It gets a lot easier after the first part has been produced.

 

We used to spend about 3 weeks waxing a new mold before making the first part - 2 or 3 coats per day. Used Maguires carnauba wax mostly. But we did some fairly intricate and deep draft parts with minimal taper. After 100 parts or so, you could start to reduce the coats between parts down from 6 to 4 or so. This was in a steel building in south Florida, so we had nice curing temps of 125 F after about 10 am.

 

I was taught to always use pva on the first part, apparently helps cure the mould and cooks the wax in on the first cycle. Pva makes crummy moulds shine and downgrades shiny moulds but safe✨
Jeff