Why applying a high gloss Acrylic Clear Coat on our oil-based wood stain has created an inconsistent topcoat

Question

I have a raw wood table where I did the following steps:

1. Sand the wood

2. Applied a linseed oil and let it dry for 3 days

3. Applied an oil-based wood stain (semi-gloss) and let it dry for one day. The result was almost perfect till the next step.

4. Finally, today I applied this high gloss varnish product, where the problem started

Product used:

I got some areas that are glossy while other are matte (as if there is dust over it). I applied the clear coat varnish when the temperature was 18C. I shook the paint cane for around 3 minutes. Now what is done is done. I am planning to sand the clear coat back and try to apply another coat.

Why did I get this inconsistent surface? Where is the problem exactly so I can avoid it on another projects and when I apply the second coat on my current project?

Here is the result I currently have:

Here is the wood stain I applied:

Answer 1

There are multiple reasons this sort of thing can happen, one of the major ones being some inconsistency in the spraying (e.g. changes in angle or distance) or spraying from slightly too far away (only a few centimetres, about 1-2", can be the difference between a good result and a slightly 'dry' or dusty looking surface).

Getting a good result with spray-applied finishes is not easy on large, flat surfaces and even more difficult with cans than with a spraygun. As a result not a few people consider that spraycan finishes are more for small projects than for large.

But there is another major cause of inconsistent gloss and that is simply when there is not enough of the finish built up on the surface yet. You've only applied one coat so far and that is almost never enough. As a rule never expect any gloss finish applied in the typical thin and even manner to give a consistent finish after just the first coat. Often two isn't enough either; 3-4 is often the sweet spot (except for very thin finishes which require more).

So, as a result I would expect this to look a lot better after you've applied the second coat, although it may require a third and possibly even a fourth for a really uniform finish.

Some tips:

• Warm the can. Shake the can well, then place it in hand-hot water — you should be able to comfortably keep your hand in the water, so think more bathwater than tea or coffee.

  While 18°C is a perfectly reasonable temperature to apply most spray finishes you generally get better results if the can is at a slightly higher temperature. Warming does two things: it reduces the viscosity of the liquid and (slightly) increases the pressure. Together these produce much better flow and atomisation of the finish.

• Once warmed, dry the can well and then shake it some more.

• Make sure you start spraying with the nozzle pointed away from the workpiece and stop spraying once it is past the other end.

• Remember to periodically shake the can during the spraying operation (every 30 seconds or so).

• Aim to slightly overlap each pass, but if you leave a slight gap don't go back over it with another coat to attempt to rectify the problem. Rely on the next coat to catch up with any missed spots.

• Spray technique tip

  

• Do not sand between coats unless absolutely necessary. Most finishes don't require it, and it always runs the risk of sanding through to the wood. If you do have to 'sand' use quite fine abrasive (320 or finer) and very gentle pressure, and take particular care along edges and at the corners.

There are many guides to spraying online if you want to delve deeper into good spraying technique.