Use of equity in long term investment

[John Pool]

The question of using equity in a primary residence has come up quite a few times in the forums, but I think my question is a bit different (but not by any stretch creative). 

We own an investment property valued at around $520k with about $210k left on the mortgage. This property has been consistently rented for the past seven years and we would like to keep it (thus the long term nature of it). 

We are interested in finding some way to use the nice amount of equity in this investment property to fund purchases of multiple additional long term rental properties. 

I have seen a few posts related to using non-primary residence HELOCs. That seems like a good option to come up with down payments for two more properties. I wonder if that would work to use that money for down payments then get conventional mortgages for the remainder of the cost of the additional properties? 

Does anyone else have experience using equity in a current investment property to build their portfolio and buy additional properties?

We are just getting serious recently about getting additional properties so any/all advice would be appreciated (we both have full time jobs, but we want properties in our stable of other investments as a diversifier). I have been absorbing the content of this site like crazy and will continue to do so. And please forgive me if this has been covered elsewhere. Sometimes a site has SO much great content it is easier to post and get replies than piece together bits of the information. 

Also, if anyone would like to connect please feel free. I would love to connect with folks in the northern VA area who have advice to give.

Thanks!