UP Pre gel vs. PU Coatings

Comparison of UP-Vorgelat and PU coatings, with a final conclusion.

Fig. 1 Masking the mold with solvent-resistant, sharp-edged adhesive tape, e.g. Tesa 4440
Fig. 2: Blue paint sprayed into the mold
Fig. 3: Tesa tape removed for next color shade

Fig. 4 Sprayed red
Fig. 5: After a short period of gelation, very thinly back-molded in white to achieve better opacity in red
Fig. 6 Closing the mold

Fig. 7: Demolded tail unit underside
Fig. 8. demolded tail unit upper side, trimmed, rudder cut out
Fig. 9. demoulded tail unit underside, trimmed, rudder cut out

Advantages of PU paint:

1. significantly better coverage than UP-Vorgelat, therefore lighter component weights can be realized.
2. less odor development than UP-Vorgelat.
3. all RAL colors available, except fluorescent colors
4. no time-consuming mixing in of color pastes
5. better UV stability than UP-Vorgelat
6. significantly longer storage stability than UP-Vorgelat

-----------------------------------------------------------------------------------------------------------------------------

Advantages of UP-Vorgelat:

1. slightly better chemical resistance if only small amounts of color pastes are mixed in, except for finished UP-Vorgelat white
2. better scratch resistance than PU lacquer
3. better bonding to the epoxy resin system, even after a longer curing time

-----------------------------------------------------------------------------------------------------------------------------

Conclusion:

Overall, the advantages of PU lacquer outweigh those of UP-Vorgelat. The PU lacquer system from Lechler works well:
Isolack PUR 29144 Lechler, Hardener 29344 Iso Hardener Lechler.

Colors such as red, yellow, orange etc. should be back-fogged with white for better coverage and brilliance.

Gun nozzle sizes of 0.8 mm-1.5 mm have proven to be effective. The spray pressure depends on the gun, but can be slightly higher for better coverage due to finer atomization. The smaller the nozzle, the finer the atomization and the more paint mist and vice versa. Of course, this only applies to one type of spray gun and cannot apply to all spray guns in general.  

To avoid flow and separation problems, it is important to mist the paint very thinly at first, allow it to set and then continue painting in thin coats.

The best processing temperatures are 20 - 22 °C.

The addition of thinner is not necessary with this type of paint. Accelerator additives are possible, but overdosing will embrittle the hardened paint and cause bonding problems with the epoxy.

It is very important that the lacquer is cured and most of the solvent has evaporated from the system before laminating. Otherwise there may be demolding problems, as the release agent can be attacked by the solvent. However, this problem is also heavily dependent on the release agent. This should be as resistant as possible to the solvent. The chemical resistance of the mold resin to the solvent in the paint is also important.

This mold resin system works for the coating system described: Mould Resin P + Hardener P-25 (20 min).
This release agent works very well for PU coating systems: Release agent Semiperm™ Monofilm  .
Works very well as a cleaning agent for the spray gun: Acetone.

As the bonding to the epoxy resin system is not based on cohesion but on adhesion, it is optionally advisable to sand the paint using a sanding fleece, at least in the areas that are machined on the component.

A very elegant method of storing the paint and mixing it up easily would be so-called stirring lids. These are also used in professional paint mixing benches. They can be used with a simple 3-D pressure adapter, also with a cordless screwdriver, see photo.