Primer Differences???
Thread Starter
Burning Brakes
Joined: Aug 2011
Posts: 983
Likes: 35
From: Redding California
Primer Differences???
What is the difference between a Polyester 2k primer and a Urethane 2k primer? And which one is the best to go over a stripped 68 coupe? The reason I ask is that I have Urethane 2k high build on hand. But I've never used it on fiberglass? I've read on the forum here that a lot of people use Slick Sand; and it's a Polyester primer? I realize that this is the first "coating" that's going to be applied to my freshly stripped 68 and I want it to be the correct one. Thanks in advance to all who chime in... And what primer do you recommend?
Melting Slicks
Joined: Aug 2006
Posts: 2,607
Likes: 374
From: Oceanside CA
You will have several opinions coming your way. Porchdog recommends the glass be epoxy primed 1st with SPI epoxy primer. DUB recommends gel coat first. Others will put the urethane 2K over the glass and others a polyester over the glass. DUB and Porchdog are both pros and they are both right. You will need to weigh your options and follow through. Just be careful at each step remembering the most important step is the one you are doing right now. Good Luck and have fun.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,753
From: Charlotte NC
LONG REPLY:
Dave is correct. For me...I would apply a gelcoat. This process takes time and requires a paint gun with a fluid tip large enough to push and atomize the gelcoat effectively...at least a 2.8.
Now this same gun can be used to apply a polyester primer...which is also quite thick.
The polyester primer I use, uses MEKP ( Methyl Ethyl Ketone Peroxide) as the catalyst to harden the primer. Which is the same ingredient that is use to harden polyester fiberglass resin. While urethane primer does not use MEKP...it uses a different chemical...due to how it is formulated. I also add some acetone to my polyester primer mix during the stages of application to aid in thinning it a bot and getting it to lay down and have a smooth texture...which makes sanding go faster.
YES...I use polyester primer...but I DO NOT apply it first. I apply a gelcoat....then after it has been blocked down...I apply polyester primer on top of that to really get the body straight as possible.
Urethane 2K is a product I use...but not in this situation. I usually use in on NEW Corvettes that often times do not require the need for such high build primers like older fiberglass/SMC Corvettes.
SORRY...I can not reply on which is easier to sand wet or dry...because I am not concerned about "how easy" a product sands. I am more concerned about how it performs and lasts. For me all products sand easy if you understand their properties and know when you can go in and sand BEFORE it totally "rocks-up". Some products are what they are...and takes time to sand...but that is the time I like the most because that is where all the previous hard work is starting to become refined and perfected and can be honed to perfection.
IN MY OPINION: Gelcoat and polyester primer are very close to one another in regards to how they can fill and protect a panel. The polyester primer has a lot of talc in it that is the component that gives it the high build/filling capability....and it is used in many primers...and makes it somewhat easy to sand. Gelcoat is made out of other ingredients and when cured provides a much better shell to protect the paint form issues coming up from underneath. Gelcoat is only good if it has been applied to achieve a minimum film thickness of 20 mils AFTER it has been sanded. If you do not get enough on it will defeat what the gelcoat can do for you. Much like if you were to pour a driveway in concrete...and only pour 2 inches...it will crack because it does not have enough strength due to being so thin.
I get into discussions with people that feel that using polyester primer is fine...and I say..."Do what you want." But..for me...doing these cars for people....I feel "sure as shooting"...if I by-pass a critical step to save time and or money....and a problem occurred....I would be kicking my self in the "backside" because I changed a procedure that I have followed for so long with NO PROBLEMS. Working on these chrome bumper Corvettes...I feel that they deserve the best possible...but that is just my opinion.
One thing you have to remember...and this is advice can and possibly will pay off if you take heed. When I am doing a paint job. And I know that I am going to be applying gelcoat, polyester primer, paint and clear. I know how thick these layers will add to the outside surface of the car when completed. And KNOWING that these cars did not get the "special treatment" of these materials originally. That this "new" film build can effect how some parts will fit when re-assembled...IF YOU DO NOT take this added film thickness into account. SO...for example....if your headlight doors have good gaps and are flush...but these gaps are on the tight side....and you go in and gelcoat and polyester prime it...then paint and clear it...and then go and raise the headlight door.....DO NOT be surprised if you do not wipe off some paint and damage your new paint job. I ALWAYS adjust the gaps on all moving parts of the car to compensate for the materials that I am going to apply on the bare fiberglass. And if you do not shoot a test panel ( because this is your first "rodeo" on this)....then that is your fault if you have a problem. I have talked to many people and have advised them to take this issue into account...and if they are the type where they want the gaps to be within GM body checking gap specs...FINE...But just make sure you open up these critical areas so when you make a dedicated effort to apply these "new" materials on these areas. These materials will close up the gaps and make them perfect when completed. That is how I do it and when completed the gaps are "spot-on!!!". But I am also making a dedicated effort to apply the materials to these edges to gain film build...because I am SUPER PICKY about panel edges being perfect.
I know this is more than you wanted to possibly read. But if you are getting ready to venture down a path. Not taking "things" into consideration can backfire on you...like it has for me years and years ago when I paid dearly to learn how to adjust "things to make the job get completed without a problem. Much like filing on the door lock cylinders...so the clip will engage correctly WITHOUT EXCESSIVE tension...due to the film build much greater than what GM did...and how the door lock cylinder was designed to fit on a panel that did not have this thick layer of product. BECAUSE if you do not take things like this into account...you may walk out one day and see "mooshed" paint around your door lock cylinder due to the clip is so tight...and the clear has bubbled up around the cylinder. DO NOT ask me how I know this! And you a have to allow areas where items are under tension/pressure when installed on the paint...you gave you paint enough time to cure BEFORE you attach them...which also goes hand in hand in how you painted it and if you gave the paint enough time to flash off between coats so you are not trapping solvents.
It is a BIG "daisy-chain" of events that have to be taken into account so you end up with a flawless job. And I repeat...in my opinion....and does not mean MY WAY is the ONLY WAY...because it is not. It is just that...MY WAY of doing things.
DUB
You will have several opinions coming your way. Porchdog recommends the glass be epoxy primed 1st with SPI epoxy primer. DUB recommends gel coat first. Others will put the urethane 2K over the glass and others a polyester over the glass. DUB and Porchdog are both pros and they are both right. You will need to weigh your options and follow through. Just be careful at each step remembering the most important step is the one you are doing right now. Good Luck and have fun.
Dave is correct. For me...I would apply a gelcoat. This process takes time and requires a paint gun with a fluid tip large enough to push and atomize the gelcoat effectively...at least a 2.8.
Now this same gun can be used to apply a polyester primer...which is also quite thick.
The polyester primer I use, uses MEKP ( Methyl Ethyl Ketone Peroxide) as the catalyst to harden the primer. Which is the same ingredient that is use to harden polyester fiberglass resin. While urethane primer does not use MEKP...it uses a different chemical...due to how it is formulated. I also add some acetone to my polyester primer mix during the stages of application to aid in thinning it a bot and getting it to lay down and have a smooth texture...which makes sanding go faster.
YES...I use polyester primer...but I DO NOT apply it first. I apply a gelcoat....then after it has been blocked down...I apply polyester primer on top of that to really get the body straight as possible.
Urethane 2K is a product I use...but not in this situation. I usually use in on NEW Corvettes that often times do not require the need for such high build primers like older fiberglass/SMC Corvettes.
SORRY...I can not reply on which is easier to sand wet or dry...because I am not concerned about "how easy" a product sands. I am more concerned about how it performs and lasts. For me all products sand easy if you understand their properties and know when you can go in and sand BEFORE it totally "rocks-up". Some products are what they are...and takes time to sand...but that is the time I like the most because that is where all the previous hard work is starting to become refined and perfected and can be honed to perfection.
IN MY OPINION: Gelcoat and polyester primer are very close to one another in regards to how they can fill and protect a panel. The polyester primer has a lot of talc in it that is the component that gives it the high build/filling capability....and it is used in many primers...and makes it somewhat easy to sand. Gelcoat is made out of other ingredients and when cured provides a much better shell to protect the paint form issues coming up from underneath. Gelcoat is only good if it has been applied to achieve a minimum film thickness of 20 mils AFTER it has been sanded. If you do not get enough on it will defeat what the gelcoat can do for you. Much like if you were to pour a driveway in concrete...and only pour 2 inches...it will crack because it does not have enough strength due to being so thin.
I get into discussions with people that feel that using polyester primer is fine...and I say..."Do what you want." But..for me...doing these cars for people....I feel "sure as shooting"...if I by-pass a critical step to save time and or money....and a problem occurred....I would be kicking my self in the "backside" because I changed a procedure that I have followed for so long with NO PROBLEMS. Working on these chrome bumper Corvettes...I feel that they deserve the best possible...but that is just my opinion.
One thing you have to remember...and this is advice can and possibly will pay off if you take heed. When I am doing a paint job. And I know that I am going to be applying gelcoat, polyester primer, paint and clear. I know how thick these layers will add to the outside surface of the car when completed. And KNOWING that these cars did not get the "special treatment" of these materials originally. That this "new" film build can effect how some parts will fit when re-assembled...IF YOU DO NOT take this added film thickness into account. SO...for example....if your headlight doors have good gaps and are flush...but these gaps are on the tight side....and you go in and gelcoat and polyester prime it...then paint and clear it...and then go and raise the headlight door.....DO NOT be surprised if you do not wipe off some paint and damage your new paint job. I ALWAYS adjust the gaps on all moving parts of the car to compensate for the materials that I am going to apply on the bare fiberglass. And if you do not shoot a test panel ( because this is your first "rodeo" on this)....then that is your fault if you have a problem. I have talked to many people and have advised them to take this issue into account...and if they are the type where they want the gaps to be within GM body checking gap specs...FINE...But just make sure you open up these critical areas so when you make a dedicated effort to apply these "new" materials on these areas. These materials will close up the gaps and make them perfect when completed. That is how I do it and when completed the gaps are "spot-on!!!". But I am also making a dedicated effort to apply the materials to these edges to gain film build...because I am SUPER PICKY about panel edges being perfect.
I know this is more than you wanted to possibly read. But if you are getting ready to venture down a path. Not taking "things" into consideration can backfire on you...like it has for me years and years ago when I paid dearly to learn how to adjust "things to make the job get completed without a problem. Much like filing on the door lock cylinders...so the clip will engage correctly WITHOUT EXCESSIVE tension...due to the film build much greater than what GM did...and how the door lock cylinder was designed to fit on a panel that did not have this thick layer of product. BECAUSE if you do not take things like this into account...you may walk out one day and see "mooshed" paint around your door lock cylinder due to the clip is so tight...and the clear has bubbled up around the cylinder. DO NOT ask me how I know this! And you a have to allow areas where items are under tension/pressure when installed on the paint...you gave you paint enough time to cure BEFORE you attach them...which also goes hand in hand in how you painted it and if you gave the paint enough time to flash off between coats so you are not trapping solvents.
It is a BIG "daisy-chain" of events that have to be taken into account so you end up with a flawless job. And I repeat...in my opinion....and does not mean MY WAY is the ONLY WAY...because it is not. It is just that...MY WAY of doing things.
DUB
