I stumbled across the question Transparent resin to fill small acrylic plastic container?
At first glance the question seemed to be off-topic as asking for a product recommendation, so I started writing a comment linking to our help topic What topics can I ask about here?. While doing so, I felt more and more like nitpicking, because this question falls sqarely into a grey area between on-topic and off-topic...
The main section of the question is:
Some concerns, in rough order of importance:
1) Shrinkage. I want the resin to entirely fill the container once it has cured, with no gaps. If it shrinks and pulls away from the edges this would be bad.
2) Crazing or other chemical reactions with acrylic. I have heard that polyester resin (or maybe just the hardening catalyst?) reacts with acrylic plastic to cause crazing (tiny cracks) on the acrylic surface. This would be bad in my application. I am not sure if epoxy or other resins have this problem.
3) Ultraviolet transparency. Ideally I would like the resin to be completely transparent to ultraviolet, although some translucency (browning?) may be OK. (The glow-in-the-dark powder needs to charge.)
4) Transparency. I would like it to be as clear as practical. It is going to become somewhat translucent when I add the glow-in-the dark powder, so I would prefer to at least start fairly clear.
I would appreciate suggestions as to a resin or process that could satisfy these criteria.
Our help page states that questions about "Material selection and usage" and "Attributes of different media and how they interact" are on-topic, while "Product and service recommendations" are off-topic.
The OP didn't explicitely ask for a product or brand recommendation, but any answer could easily fall into into that category, depending on the person writing the answer.
I can see several more questions falling onto that grey-area searching for "kind material", like
- Thermochromic material for low temperature?: 2 answers containing product recommendations
- What kind of paints should I use to color / paint on bright wood material?: 1 answer containing general material recommendations
- From what material/s I need to create a mold so that I can pour in it safely any kind of molten plastic?: 2 answers containing both product and material recommendations
My question is:
When is a question asking for product recommendations and therefore off-topic? What exactly is the difference between "material selection" and "product recommendation"? And do we need to clarify the difference in our help page?