I've never really had a problem with foam-core, but I do think that depends on humitity.

Gator Board is sent from God when it comes to backing. Its about as strong as ply-wood and does not bend.

For A sell price, keep it to 15% - 25%. Think about your target market and consumers, and how much they would pay. Also consider how much you would pay for an image that size and quality.

Depending on your clients, you could charge as much as you want.

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