Gelcoat related questions

[DeepBlue2010]

I always used wax at the final coat of the gel to grantee curing and I had no problem with it. Recently, I learned from ondarvr replies to some gel/resin related questions that the use of wax could be a little problematic. Granted,, most of the scenarios he shared with us are relevant to factories and mass production of boats but there are few comments he made that got me thinking. For example, he mentioned that continuous rolling (if the gel applied by a roller) can take the wax away from the surface. Also he mentioned that if the temp is hot enough, the gel will cure before the wax gets a chance to migrate to the surface. On the other hand, if temp is cold, wax will solidify and will not be distributed evenly in the gel.

Ok, so maybe using wax is a little tricky to get right. So what is the alternative? PVA

But PVA is not trick free either. I read a lot that PVA needs to be applied at a certain point of time, what is this point of time? Moreover, how to maintain this point of time if we are spraying/applying gel to large area. For example, assuming that this point of time is when the gel starts to tack, this is just an assumption to make the point but I am not sure it actually is the right time since I never used PVA, and assuming we are spraying a large hull; the first area sprayed will start to tack while we are still spraying the rest of the hull ! So this "best point of time" will be a moving target and will vary by area; How can we handle this.

Another question is regarding dealing with scratches. I don't prefer to wet sand/buff them away because this will reduce the longevity of the gel. The whole thickness of the gel is less than 1 mm or less than 30 thou. Each regular buffing and shinning session takes 1-2 thous out of this - if not more - depends on how aggressive the compound that was used. With that said, if we add wet sanding to the mix, the service life of the gel will be significantly reduced.

I am thinking, instead of sanding them down, why we don't build them up by adding gel paste and sand that down. Is there anything wrong with this?!

If the answer is no, I have few questions

1) Why the gel repair shops don't do it this way and insist on sanding the scratch down?
2) What is the correct recipe of making gel past. Few years ago, I ordered Spectrum past and dealt with it. I also made my own by adding cabosil to the gel but they were not the same. The paste from Spectrum was very sticky like it has glue into it. the closest I can describe it is cold honey. Adding cabosil reduces viscosity but it doesn't add this glue effect to the gel. Is there a trade secret we are not aware of?

3) No matter what filler and what not will be added to the gel, should we catalyze based on the quantity of the gel only or the sum of everything? In other words, should we catalyze before or after adding any additives whatever they will be?

 

[ondarvr]

Those are some very good questions, most people don't get that deep into the details.

One other issue you can run into with wax is when using acetone to thin the gel coat. You won't see acetone recommended for thinning gel coat very often because while small amounts of 2-3% typically doesn't create problems, more can though. Acetone does two things, it reduces the viscosity so the gel coat will spray better, and it acts as an inhibitor to slow down the gel time, to work correctly it needs to evaporate out of the mix, if it doesn't the cured gel coat may be soft, not weather well, be less water resistant, plus a few other things. Another issue with acetone and wax is related to how fast it evaportes, as you all know, acetone feels cold due to how fast it evaporates. This rapid cooling can cause the wax to solidify either when you add the acetone to the mix, or during the spray proccess, it will form small chunks that can cause tiny pits in the finished spray job. And when it forms these small solidified blobs there isn't enough wax left to cover the surface, so it can remain tacky.

Wax can also change the color of gel coats, dark blues, greens and a few others are very sensative to what, and how much is added to them, and since getting a good color match when patching gel coat can be difficult, all the additives can either help or hurt. Wax tends to make the color a little lighter, styrene and other clear solvents tend to make the gel coat look darker, so adjusting the mix can fine tune the color. Acetone doesn't follow the rule, it can mess with the color, but it frequently makes it lighter.

Now the good and bad of PVA.

Since PVA doessn't mix with the gel coat, it doesn't affect the color or other properties of the gel coat, which is good, but it isn't used that much because there are other limiting factors. Gel coat can cure slowly when sprayed in a thin film, especially if the ambient temps are low, even at 70* it can be slow. Many of the additives designed for adding to gel coat when patching will speed up the gel time so the repair can be completed quicker and also so the gel coat cures well enough to be weather and water resistant. The problem is most of these patching products have wax in them, PVA and wax don't play well together, PVA tends to not lay down well over wax, it separates and beads up, so it doesn't do anything but make a mess.

Using PVA also adds a step, after cleaning the gel coat spray gun you need to spray the PVA, which can take time depending on the size of the spray job, then clean that spray eqiupment. PVA can be very corrosive, so it can't be left in the spray gun, they used to sell spray guns with many plastic parts for spraying PVA.

PVA also makes a mess, when spraying large amounts the area will get covered by the overspray, the PVA will create a green fog in the room that can contaminate other surfaces and cause problems. The results can vary with humidity levels too, it's water soluble, so in areas with high humidity it may not dry completely, which can complicate things.

The correct time to spray PVA is just a few minutes after the gel coat is applied, for the best results it needs to be applied before the gel coat even begins to cure, you don't want the cure proccess to start without the gel coat surface being sealed off from air. You don't want to wait and try to time it as begins to cure. The reason people wait is because if you spray PVA too soon there are still solvents and other things evaporating out of the gel coat, this can cause the PVA to not lay down well sometimes. Doing a very large part it can cause problems because the first part of the spray job may be curing while you're still spraying, so you may miss the correct time frame for application.

Overall, most people use wax when doing a repair, it can be a little quicker and easier than PVA. Not that PVA doesn't work, and it does have it's place, it just isn't used that much.

 

[ondarvr]

On to the scratch repair question.

As mentioned above, getting the color correct on a repair can be difficult, it can be difficult even in production when using the same batch of gel coat to make the boat and do the repair, so doing a repair a decade later after the gel coat has faded and not even using the same brand of gel coat can be much harder. Repairs also tend to become more noticable over time, so while you may get it close at the time, it may age differently than the surrounding area and stand out like a red flag later.

It also takes much more time to repair scratches if you apply gel coat, it adds steps, plus you may need to do it a couple of times to get the loook right.

So when deciding how to do a scratch repair you need to decide which method is more likley to give a long term hassle free, good looking repair. Most of the time it means just sanding out the scratches and buffing it.

Making putty (paste) is normally done with just Cabosil, things like talc can be added to make it creamier and easier to use, but they don't get used that often gel coat. If you use a mixer in a drill motor you will get a much smoother and more consistant mix, plus making more at one time makes it easier. It also takes a little while for the gel coat to fully wet out the Cabosil, so the consistancy will be better the next day, or even a few days later, than it is right after you mix it in. While adding catalyst prior to adding the Cabosil would be the best method, it isn't practical most of the time because the gel coat will begin to gel in just a few minutes, so you don't have much time to get the cabosil mixed in correctly.

Catalyst amount should be based on the amount gel coat (or resin), not the total amount of putty, but the Cabosil doesn't add that much to the total volume, so it's not that big of an issue.

 

[DeepBlue2010]

Thanks for taking the time to answer my questions thoroughly. Much appreciated. I should have thought of the color matching issue. My two boats are white and GCS here in Seattle made my life much easier when it comes to color matching