Stripe coating is the single most important thing you can do to prolong the life of your coating system, and the best bit is it will cost you absolutely nothing just a bit of time.
Stripe coating is a must do process in marine and civil engineering projects as part of the paint coatings specification, no stripe coat no pass. So what is it? A stripe coat is an additional coat of paint applied to the edge of steel work prior to the full coat being applied brush or spray.
Brush Strip Coating
When painting the underside of a floor pan or any part of a steel structure that has many edges and angles, captive nut heads and bolts rivet heads, welds, brackets etc. Using a round sash brush you apply a stripe of paint along all of the edges of the steel along with all of the other items mentioned. You do not apply any further than the edge or the actual nut or bolt head. Why ?
When you apply paint by brush or spray to lets say a flat plate of steel with sharp edges in one coat the surface tension caused by the coating drying will pull the paint from each direction away from the sharp edge, this in turn reduces the dry film thickness of the coating on that edge by a huge amount. The reduced paint film on the edge will be far thinner than the manufacturers specification requires and will cause an early failure of the paint coating. To reduce the risk of this happening we stripe coat all of the edges or difficult parts of the assembly with a brush coat to all of the edges of the assembly and also any areas that would have been hard for a spray gun to get into. When the stripe coating is dry you then apply the full coat to the assembly including the edge again this ensures that the coating is up to specification in these vulnerable areas.
Spray Stripe Coating
Using a gravity spray gun it is possible to reduce the width of the spray pattern to a round spray this can then be used like the brush to stripe coat all of the edges of the car body work prior to the application of the full coat. I am convinced that a lot of the problem’s manufacturers had around the turn of the century was due to the spray robots not programmed to stripe coat. On many coating failures we have examined from the worlds manufacturers you can clearly both see and measure the dft (dry film thickness) of the finished coating and in all cases of failure it was less than the specified dft of the paint manufacturer. One large prestige vehicle manufacturer was having trouble with corrosion appearing of the back edge of the boot lid, the car had been sprayed by a robot. On examination there was hardly any paint on the back edge of the boot. Not wanting to have to remove the boot in the manufacturing process we advised the use of electrostatic spraying of the finish coat for these areas. This was done by hand using electrostatic spray guns, the edges where coated to these vulnerable areas prior to the robot spray and the problem was solved no more rust or corrosion. Hopefully robots are programmed to do this now.
Remove the edge
Not practical on very thin gauge steel, but if possible round the edge of the steel to be coated with a flap wheel grinder or file this will further reduce the chance for the paint to pull away as it dries, on this metal you really do not have that option and stripe coating is the only way.
Faraday Cage Effect
Have you ever tried to spray into a corner of a box, the air turbulence will swirl around in the corner and will not allow any paint to settle within the box section or corner this is called the Faraday cage effect. For box sections or assemblies that could trap or disturb air when being spray coated it is a good idea to coat inside these components prior to spraying to ensure good coverage of the paint coating.
Different colour primers and build coats
If you know that you are going to apply two coats of primer or as we are discussing a stripe coat it is a very good idea to apply the coats in different colours. This way the first coat is both easy to see and also easy to cover again ensuring full coating thickness and maximum life of the coating.
So there we have it a few tips to consider prior to painting that are going to improve the life of your coating and it only costs the time to do it.
All the best
Do it right do it once
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