spray gelcoat - hardness

[nevilleh]

Just a simple question about the hardness of gelcoat that has been thinned and then sprayed.

We have a surveyor coming in to the yard to hardness test the gelcoat on a new hull. But this hull we made longer by 3 ft, and refinshed on the outside with normal brushing gelcoat thinned with styrene and then sprayed on .

The surface was then rubbed down and flattened off so it blended in to the gelcoat.

Can anyone tell me if that surface will be softer than if it had not been thineed in the first place ?

I presume that all the styrene will still of come out and that thinning will not make it softer long term. Or am i in for a shock !

Thanks in advance !


Regards
Neville
Scotland

[ondarvr]

If he is doing a barcol test, then the results can be a little deceiving.

To accurately test the barcol hardness of gel coat you need to do a casting, this gives sufficient thickness to eliminate the substrate (the glass laminate in this case) from becoming a significant portion of the reading. The typical test is done on a casting done in the lid of a 1 pint metal can, its allowed to cure for a specific amount of time, then tested.

A 20 mil thick film of well cured gel coat over a somewhat soft laminate will give a low reading, the reverse can be true also.

Adding styrene can (may) actually increase the hardness of the gel coat, don't confuse hardness with good physical properties though. Styrene will reduce the water, weather and crack resistance of gel coat depending on how much is added.

[Herman]

... and make it turn yellow faster. Residual styrene will turn yellow.

Also, the percentage of pigment will drop by diluting.

A spray gelcoat does have very different thixotropic properties. Thin through the nozzle, thixotropic on the mould. Also inline heaters are popular on spray equipment.

Still for smaller repairs it is choosing between the bad and the ugly: Thin with styrene and properties might be lost. Thin with acetone and the gelcoat might be porous.

My best guess is that spraying thinned gelcoat through a small nozzle, and from some distance, is your best bet.

If it is know that some testing is to be done, try and make a test panel, to test if the chosen method produces acceptable results.

Like the previous poster said: Make sure you know what you are testing, the gelcoat or the laminate behind it.

A barcol hardness tester can be quite useful, once you get a feel of the numbers you should be able to obtain. Testing is quick and simple, and hardly leaves an impression. For what you get, the testing equipment is quite expensive, though. (If someone could direct me to a tester that is affordable, I would be pleased)

[nevilleh]

Thanks for the info. Our surveyor does barcol tests on the outside gelcoat which i have never really 100% thought that much information can be gained from that type of testing.

No testing is done on the inside of the laminate so surely the test can only prove how hard the layup is directly below that area of gel, or potentially how thick the gelcoat is.

Anyway, thanks for answering re styrene .

Neville