Hi Paul,
Unfortunately, with the <hr> tag restricted there isn't a way built into the forms or settings for them to create a horizontal rule in some sections but not others if you are using section headers to help you break up parts of your form. It is possible with a rich text formula field and by suppressing the field name to drop in colorized or stylized text headers to help break up a form visually but it does behave differently from an actual section header.
If being able to add in a horizontal line with more control over where they display would be helpful to you I would suggest submitting that feedback to our User Voice. User Voice is a tool used by our product team to track customer interest in new features and enhancements to existing features. The Quick Base User voice can most easily be accessed from the My Apps page in Quick Base by clicking on the orange Feedback tab that appears on the right of the page or at
http://quickbase.uservoice.com by signing in with your Quick Base credentials. There are a few existing requests around forms as well if you wanted to add your comments to one of those or add your vote to support them. Sorry there isn't an easier trick I can point you to right now.
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Evan Martinez
Community Marketing Manager
Quick Base
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