|
The Fibreglass King
|
|
|
Group: Forum Members
Posts: 69,
Visits: 652
|
Hanaldo, Very well put, your advice is always on the ball. You are an asset to this forum.....Kind RegardsThe Fibreglass King
|
|
|
|
|
Hanaldo
|
|
|
Group: Forum Members
Posts: 2.5K,
Visits: 28K
|
I do believe people have long confused the hygroscopic nature of some primers with porosity. Just because a material can absorb moisture does not mean it is porous. Again, this depends on the formulation of the paint. Primers are not intended as protective coatings, they don't have any of these qualities built into them because they don't need it. However even this, I believe, is no longer really relevant. Old style cellulose primers are likely the culprit, as cellulose is naturally hygroscopic. Hence spray painters have been coached into treating primers as non-sealed substrates, which when you are painting a car for $15,000 does make sense, you follow certain procedures. Polyester primers are a similar story, generally the rule is you ALWAYS dry sand polyester coatings and allow them to cure overnight before commencing wet sanding otherwise they absorb water. That isn't just primers though, it's any polyester coating, and it isn't due to porosity. I can understand why spray painters have this mindset about primers absorbing moisture; hell if I were spray painting my car I wouldn't leave it in primer either. But in the context of making moulds, a 2k polyurethane is not porous. It will not absorb release agent, it will not absorb gelcoat, it will not absorb anything.
|
|
|
|
|
kidpaint
|
|
|
Group: Forum Members
Posts: 99,
Visits: 676
|
Actually primer is porous. Thats why they require a top coat as a color and protective layer. You are right that paint alone wont stick to metal and the primer is there to help adhesion. This is why when metal is left in primer it can still rust. In my painting course we are always taught to never leave anything in primer esp if it is going to be out in the elements. If you are using a 2 part paint system, if the catalyst is mixed correctly then the only thing that can soften it is a paint remover. acetone or thinner once cured shouldnt have any effect.
|
|
|
|
|
Hanaldo
|
|
|
Group: Forum Members
Posts: 2.5K,
Visits: 28K
|
I think you guys are misunderstanding how a primer works. Primers don't absorb anything, they are non-porous. In fact any material that absorbs anything is a poor substrate for top coating. Primers improve the adhesion of paints yes, but that's because of the way that paints are formulated. Top coats need to be formulated to have qualities such as good gloss retention, durability and colour holdout. These qualities often mean that those paints have a reduced ability to adhere to substrates. That's where the primer comes in. Primers don't need to be able to hold a gloss or have good durability, so the formula can instead focus on qualities like good self-levelling and good adhesion to substrates. So they simply help adhesion by being an interface; they bond well to the substrate they are being applied to so that the topcoat that goes on top doesn't need to. Don't forget that to get paint to adhere to primer still requires meticulous preparation. It's all due to the fact that top coats typically have poor adhesion qualities. The ability to mould off these surfaces is somewhat unrelated. A fully cured 2k primer will not absorb Easylease or any other chemical release agent. Chemical release agents are essentially extremely thin resins (paint is also a resin) that have been desolved in a solvent. When applied to the surface, the solvent evaporates and leaves the resin behind. Keep in mind that there are a few semi-permanent release agent systems that are actually a 3-step system: primer/sealer/release. The primer is there to improve the durability of the polymers that are being applied on top of it, it doesn't mean that anything and everything is going to stick to it. Where you will come unstuck (pun intended) with 2k coatings is if the tooling gelcoat softens the coating that it is applied to. Once the coating has softened, the film of chemical release agent is compromised and you will get sticks. This is just as likely to happen with a 2k topcoat as it is a 2k primer, it simply depends on the compatibility of the paint and the gelcoat and how well cured the coating is. This is why you should always test the products that you intend to use together - not all coatings are made equal. Polyurethanes tend to have good chemical resistance in general, but some are better than others. A good way to test is to polish up your coating to the standard that you desire, and then wipe it with a powerful solvent like acetone or MEK. If the surface dulls, then it's because the solvent softened the coating and I would not recommend proceeding with the tooling gelcoat. If the solvent flashes off the surface without affecting the finish, then you can be reasonably confident that the gelcoat won't attack it. Personally if I am using a new product for the first time then I always like to do a little test with the gelcoat that I'm using as well, just for piece of mind.
|
|
|
|
|
Makerbot
|
|
|
Group: Forum Members
Posts: 8,
Visits: 126
|
VVS (26/01/2016)
Been looking into this recently and do not recommend fill in any way as it comes out of the part and sticks to the mould, can be cleaned off but adds work.
regards 2K primer I think it works well so long as its cured, ive theory that it needs post curing as such that unless its old you need to cure it to above the temperature that the tooling gel coat gets to when curing, you get a load of heat in the resin and if your paint has not been above this you get can then get problems.Good point, thanks VVS.
|
|
|
|
|
Makerbot
|
|
|
Group: Forum Members
Posts: 8,
Visits: 126
|
matthieutje65 (22/01/2016)
If possible I allways avoid primer/bondo and so on to make a mould of...I always topcoat it or clearcoat it with a 2K to make sure it demoulds well... It's in the nature of these products to absorb (normally paint) coming on top so not ideal to make a mould on with the risk of it sticking...with a topcoat or clearcoat, 2K that is fully cured!, you should never have any problems with demoulding with the chemical release agent Thanks Matthieu, more to consider for the effort/result mix.
|
|
|
|
|
VVS
|
|
|
Group: Moderators
Posts: 226,
Visits: 334
|
Been looking into this recently and do not recommend fill in any way as it comes out of the part and sticks to the mould, can be cleaned off but adds work.
regards 2K primer I think it works well so long as its cured, ive theory that it needs post curing as such that unless its old you need to cure it to above the temperature that the tooling gel coat gets to when curing, you get a load of heat in the resin and if your paint has not been above this you get can then get problems.
|
|
|
|
|
Matthieu Libeert
|
|
|
Group: Forum Members
Posts: 955,
Visits: 3.4K
|
If possible I allways avoid primer/bondo and so on to make a mould of...I always topcoat it or clearcoat it with a 2K to make sure it demoulds well...
|
|
|
|
|
Makerbot
|
|
|
Group: Forum Members
Posts: 8,
Visits: 126
|
FLD (21/01/2016)
I mould from primered surfaces (2 pack) and I find the primer takes a sprayed coat of PVA release nicely to give a gloss finish. Wax will give you a near gloss finish that's easy enough to polish up. I found easylease didn't work well on primer; it seems to soak in and not act as a good release agent. Might just be my experience. Hope this helps.Thanks FLD, that's useful to know as I was planning on using easylease but that makes sense my initial concern was about mould separation because primer is absorbent.
|
|
|
|
|
Makerbot
|
|
|
Group: Forum Members
Posts: 8,
Visits: 126
|
Warren (Staff) (21/01/2016)
Professionally sprayed primer from a 2 pack system will usually be fine once fully cured. You should get a release with any decent release agent. Ideally from a mould preparation point, it would be more convenient for a high gloss painted finish (2 pack also) as it means less work needed to finish the mould surface off to a high standard. However, if you don't mind polishing up the surface then you should be fine.Thanks Warren, that helps. Was more concerned about mould separation than finish as primer is designed to stick but I understood as long as it wasn't keyed it would be okay.
|
|
|
|